12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022

IMPACT OF WALL BOUNDARY CONDITIONS ON PASSIVE SCALARS INCOMPLEX TURBULENT FLOWS

F. Secchi
Institute of Fluid Mechanics
Karlsruhe Institute of Technology
Kaiserstrasse 10
francesco.secchi@kit.edu

D. Gatti
Institute of Fluid Mechanics
Karlsruhe Institute of Technology
Kaiserstrasse 10
davide.gatti@kit.edu

B. Frohnapfel
Institute of Fluid Mechanics
Karlsruhe Institute of Technology
Kaiserstrasse 10
bettina.frohnapfel@kit.edu

ABSTRACTmation of flow structures. For instance, the shape of the crosssection of the jets nozzles, the Reynolds number in the jets,
the spacing between the jets axes, the nozzles-to-plate distanceand the presence of a cross-flow are known to cause the onsetof different flow characteristics (Geers, 2004). For a broad
range of parameters, studies in the literature show the formation of a fountain flow as a product of the interaction of thewall jets of two sufficiently close impinging jets (Barata, 1996;
Geers et al., 2001; Thielen et al., 2003). The fountain flow
consists of the upward directed flow generated by the interaction of two opposite directed wall jets. If the distance between
the free jets axes is sufficiently small, the fountain flow caninteract with the downward directed flow of the two jets withthe consequent formation of an extended recirculating region
consisting of two large counter rotating vortical structures.
Evidently, the presence of recirculating regions can affects mass, momentum and heat transfer features in the impingement region of the flow with alterations noticeable in thenear wall region and, possibly, up to large wall normal distances.
Using a direct numerical simulation (DNS), the present
work investigates a complex jet impingement configuration
where two parallel square ducts are used to generate two incompressible turbulent jets that impinge on a flat solid wall.
The layout considered in the study allows the realization ofmean flow features characteristic of jet array configurations,or of complex flow scenarios which include recirculation effects due to flow impingement. Several passive scalar fields
are resolved within the simulation with the purpose of enlightening the influence that wall-boundary conditions and diffusion properties associated to a particular scalar field have on
the global evolution of the scalar quantity. This is relevant
to understand whether or not the behavior of temperature andconcentration scalars that can experience different wall boundary conditions can be modelled through analogy assumptions(Na and Hanratty, 2000).
Even though only incompressible and non-reacting flow
is considered, the idealized setup represents a playground useful for the statistical assessment of passive scalar and velocity

This study presents the direct numerical simulation of
two incompressible turbulent jets impinging on a solid surface.
The two jets originate from two parallel ducts of square crosssection where a fully developed turbulent flow is maintained.
Several passive scalar fields are resolved within the simulation,
as it is commonly done for the inclusion of a temperature fieldor concentration of chemical species in the flow. Transportand diffusion of scalar fields in the near-wall impingement region is influenced by the diffusion coefficient and the specificboundary condition assigned to each scalar field at the wall.
The study reveals the far-off the wall areas in which effects ofwall boundary conditions are noticeable on the mean distribution of each scalar field.

INTRODUCTION
It is not uncommon that state-of-art turbulence closure
models fail to predict important flow features when applied toflow configurations of practical engineering interest. In thisrespect, turbulent jet impingement on solid walls is a wellknown example (Dewan et al., 2012; Zuckerman and Lior,
2005; Dairay et al., 2014). While the study of an isolated impinging jet is instructive for understanding the physical mechanisms at play in the near-wall heat transfer process (Dairay
et al., 2015), most practical applications exploit arrays of several jets arranged in various geometrical layouts (Viskanta,
1993). The existence of multiple jets complicates greatly the
resulting mean flow field and, thus, further extends the challenge of reproducing such complex flow features using RANS
or even LES frameworks (Geers et al., 2008; Thielen et al.,
2003; Draksler et al., 2019).
The impingement of an isolated jet generates a wall jetthat develops along the impingement plate. When multiple
impinging jets are considered, the interaction between the wall
jets produced by the multitude of the impinging jets can provoke a drastic modification of the resulting flow field. In such acase, many factors and geometrical parameters govern the development of the flow and are decisive in determining the for-1

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
Table 1. Passive scalar fields parameters. Columns “Jet 1”
and “Jet 2” refer to block-profile inflow boundary conditions
applied to the two jets. D and N represent Dirichlet and Neumann boundary conditions respectively.

Figure 1. Sketch of the flow configuration.

fields, thus providing a rich data-set valuable for the development and testing of turbulence closure models of interest.

φi

Jet 1

Jet 2

φ1

0

1

φ =0

(D)

1

φ2

0

1

φ ,n = 0 (N)

1

φ3

1

0

φ ,n = 0 (N)

1

φ4

0

1

φ =0

(D)

0.5

φ5

0

1

φ ,n = 0 (N)0.5Wall BC

Pe/Re

FLOW CONFIGURATION
The investigated flow configuration is sketched in figure 1. The turbulent jets originate form two parallel ducts ofsquare cross section through which a fully developed turbulent
flow is maintained. From the computational perspective, suchinflow boundary condition for the jets is realized by performing a precursor simulation of two fully developed turbulent
duct flows at constant flow rate where periodic boundary conditions are applied in the streamwise direction. The two ducts
are separated by an interface of finite thickness h ≈ 0.03D and
the impingement plate is located at a distance of 2D from thejets exit section. Here D represents the side length of the crosssection of a duct. The flow is governed by the incompressible
Navier-Stokes equations:

∂uj
=0
∂xj

(1)

1 ∂ 2 ui
∂ ui
∂ ui
∂p
+uj
=−
+
∂t
∂xj
∂ xi Re ∂ x j ∂ x j(2)

to retain very high order spectral accuracy in the solution. In
the context of the SEM, the governing equations are first recast in their weak form. Then, the discretisation of the functional space of the solution and the test functions is achieved
through the classical Galerkin approximation. In the present
work, the PN − PN formulation is used. Within this framework, N th -order Lagrange polynomials interpolants on GaussLobatto-Legendre (GLL) quadrature nodes are used to approximate both the velocity and pressure spaces. Time integrationof the discretized equations is performed using an implicitexplicit scheme (BDFk-EXTk, backward differencing and extrapolation of order k = 3). In particular, the linear terms aretreated implicitly, while non-linear terms are treated explicitly.
Over-integration is used for the dealiasing of the non-linear
terms. At each time-step, the discretized problem is split into
one Poisson problem for the pressure and one Helmholtz system for the velocity.
In the framework of Nek5000, equations 1, 2 and 3 areintegrated numerically within a Cartesian coordinate system(x = x1 , y = x2 , z = x3 ). The absence of statistically homogeneous directions in the flow configuration only allows theacquisition of time averages of flow variables. Results shownin the present work have been averaged in time for approximately 60 D/Ub . Further acquisition of data did not produce
sensible changes in the computed statistics; in particular, a difference of less than 3% was observed in the peak values of theux velocity component in the x − z plane at any x location. In
the subsequent sections of this work, an over-line (·) will be
used to denote time averaging, while a prime superscript (·)′
will indicate the fluctuations.
The computational meshes for the jets and the precursor ducts domains consist of, respectively, ≈ 3.86 · 106 and≈ 0.93·106 elements. A grid independence study of the results
has been performed by varying the polynomial order used forthe representation of the solution. The largest discrepancies
between the 3rd and the 5th order solutions were observed in
the mean velocity field at large distances from the jet axes (i.e.
| x/D |≥ 6). At these locations, the deviation of the computed
maximum ux velocity component on the x − z plane amounted
to approximately 6%. Nonetheless, in the region of interest of
the flow, i.e. −4 ≤ x/D, y/D ≤ 4, the greatest discrepancies in
the mean ux velocity component between the two order solutions amounted up to less than 2%. The results presented inthis work show the 5th polynomial order solution.

where ui , xi , t and p represent, respectively, the ith fluid velocity component, the ith coordinate direction, the time andthe pressure (the equations are valid for (i, j = 1, 2, 3) andsummation over repeated indices is implied). Equations 1
and 2 are made dimensionless using the mean bulk velocity
Ub in the ducts and the length D. The Reynolds number is
Re = Ub D/ν = 10 000, where ν indicates the kinematic viscosity of the fluid.
N = 5 passive scalar fields are resolved within the simulation through the numerical integration of transport-diffusion
equations:1 ∂ 2 φk
∂ φk
∂φ
+uj k =
∂t∂xj
Pe ∂ x j ∂ x j(3)

In equation 3, φk denotes the kth passive scalar field (k =1, ..., N), and Pe denotes the Peclet number. Hence, the ratio Pe/Re corresponds to the Schmidt or Prandtl number for aconcentration or temperature scalar, respectively. The parameters of the simulated scalar fields are collected in table 1.

NUMERICAL METHODThe DNS is performed using the flow solver Nek5000(Fisher et al., 2008-2021) which is based on the spectral element method (SEM) of Maday and Patera (1989) and allows

212th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
RESULTS
Mean velocity field

(a)
z/D

Mean x and z velocity components distributions in theplane y/D = 0 are reported in figure 2. Streamlines depictedin the figure evidence the formation of a large recirculating
bubble consisting of two counter-rotating vortical structures
between the two jets. The depicted flow structure is consistent
with numerical and experimental findings of arrays of impinging jets (Geers et al., 2004, 2006), even though in the present
case the small spacing between the two jets sets off a much
more visible recirculating zone. The latter is formed after the
impingement of high momentum fluid coming from the twoparallel free jets. The impingement deflects the two jets in two
opposing directions and the subsequent interaction of the two
deflected jets produces a strong upward directed flow. The upward motion of the fluid is strongly damped by the downward
directed free jets and, ultimately, the fluid gets entrained backto move toward the impingement plate. The resulting mean
flow consists of the two large counter rotating vortical structures observed in figure 2.
The recirculating region of the flow is also characterized by strong turbulence intensities relative to other regionsof the flow. The distribution of the turbulent kinetic energy′2′20.5(u′2x + uy + uz ) in the plane y/D = 0 is shown in figure
3. The turbulent kinetic energy reaches its maximum value inthe recirculating region and has a secondary peak located very
close to the wall at x/D = 0. The free shear layers of the free
jets, the mixing layer between the jets and the outer shear layers developed along the impingement plate show values of the
turbulent kinetic energy that are roughly 50% of the maximum
values of the turbulent kinetic energy reached in the recirculating region.
The presence of the recirculating region has significant
effects on the structure of the flow. Because of the bulging induced by the two vortical structures, the free jets are inclined
towards opposing directions and their mean axes are not parallel anymore near the impingement region. Consequently, thelocation of the stagnation points on the impingement plate is
not placed at the intersection of the free-stream jets axes with
the impingement plate, but it is shifted slightly outwards. Selected contour lines of the pressure field in the near impingement region are reported in figure 4. In the same figure, two
black dashed lines represent the axes of the two undisturbed
jets. It is clear from the figure that the maxima of the pressure
field, which correspond to the locations of the two stagnation
points, are not located exactly underneath the jets axes, butthey are slightly shifted toward the outside region.
The development of the flow towards the positive and negative x directions does not appear to be significantly affectedby the recirculating region. On both sides of the two jets, thedeflection of the flow produces a wall-parallel flow that has
characteristics very similar to those of three-dimensional wall
jets (Kakka and Anupindi, 2021). Selected contour lines of theux velocity component in the y − z planes at x/D = 1, 2, 3 arereported in figure 5. It can be seen that, as the flow develops
in the downstream direction, the wall jet spreads in the y direction, while the region of maximum velocity gets flattened.
Naturally, a symmetric behavior is observed for the development of the flow in the negative x direction. On the other hand,
the wall jet that develops in the y direction after the jets impingement is significantly affected by the recirculating region
that exists between the two jets. Selected contour lines of theuy velocity component in x − z planes at y/D = 1, 2, 3 are displayed in figure 6. The contour lines evidence a bulging region of high speed fluid located far away from the wall (i.e.

2
1.5
1
0.5
0
−4

−3

−2

−1

0

1

2

3

4

1
0.5
0
−0.5
−1

x/D

z/D

(b)
2
1.5
1
0.5
0
−4

−3

−2

−1

0

1

2

3

4

1
0.5
0
−0.5
−1

x/D

z/D

Figure 2. Time averaged mean velocity distribution in theplane y/D = 0. (a) ux /Ub velocity component; (b) uz /Ub velocity component. White lines represent streamlines.

2
1.5
1
0.5
0
−4

0.2
0.15
0.1
0.05
0
−3

−2

−1

0

1

2

3

4

x/D

z/D

Figure 3. Time averaged turbulent kinetic energy in the planey/D = 0.

2
1.5
1
0.5
0
−4

0.7
0.6
0.5
0.4
0.3
−3

−2

−1

0

1

2

3

4

x/D

Figure 4. Time averaged contour lines of constant pressurein the plane y/D = 0. Black dashed lines represent the jetsaxes.

z/D ≈ 1 at x/D = 0. For increasing y distances, this region ofhigh speed fluid has the tendency to detach from the underlying developing three dimensional wall jet.

Passive scalar fields
The recirculating region between the two jets has a profound impact on the structure of the mean flow and, consequently, it is expected to affect also the mean distribution ofpassive scalar fields. As a primary effect, the recirculating
flow produces an intense upward motion in the near wall impingement region close to x/D, y/D = 0. In this portion of thedomain, the upward directed flow can advect the boundarycondition-informed passive scalar field away from the walland, in this sense, spread the information of wall boundary
conditions at large distances from the wall. The spiralling pattern followed by fluid particles entangled in the recirculating3

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
(a)
0.6
0.5
0.4
0.3
0.2
−3

−2

−1

0

1

2

3

z/D

z/D

(a)
2
1.5
1
0.5
0
−4

4

2
1.5
1
0.5
0
−4

0.6
0.5
0.4
0.3
0.2
−3

−2

−1

y/D

−3

−2

−1

0

1

2

3

4

2
1.5
1
0.5
0
−4

−3

−2

−1

−1

0

1

2

3

z/D

z/D

4

0

1

2

3

4

(c)
0.6
0.5
0.4
0.3
0.2

−2

3

x/D

(c)

−3

2

0.6
0.5
0.4
0.3
0.2

y/D

2
1.5
1
0.5
0
−4

1

(b)
0.6
0.5
0.4
0.3
0.2

z/D

z/D

(b)
2
1.5
1
0.5
0
−4

0
x/D

4

2
1.5
1
0.5
0
−4

0.6
0.5
0.4
0.3
0.2
−3

−2

−1

0

1

2

3

4

y/D

x/D

Figure 5. Time averaged contour lines of the velocity component ux /Ub . (a) plane y/D = 1; (b) plane y/D = 2; (c) planey/D = 3.

Figure 6. Time averaged contour lines of the velocity component uy /Ub . (a) plane x/D = 1; (b) plane x/D = 2; (c) plane
x/D = 3.

region of the flow and the high turbulence intensities observedin this region are also expected to strongly influence the local
advection and diffusion of passive scalar fields.
To address these points, several passive scalar fields havebeen included in the DNS. Each simulated scalar field differs from the others because of the prescribed inflow and wallboundary conditions and diffusivity properties. The parameters assigned to all the simulated scalar fields are reported intable 1. In all cases, the inflow profile consists of a uniform
distribution on the exit section of a jet. For each scalar field,
the constant is set equal to 1 on one jet and 0 on the other.
The columns “Jet 1” and “Jet 2” in table 1 refer to the inflow
profiles assigned to each scalar field. An example of time averaged distributions of passive scalar fields φ1 and φ3 in the x − zplane at y/D = 0 is displayed in figure 7.
Sampling of data during the simulation at specific locations provides with time-resolved data useful for assessing theinfluence of wall boundary conditions and diffusivity properties on the evolution of passive scalars. Scatter plots of threedifferent scalar pairs are reported in figure 8 for data sampled
at x/D = 0.022, y/D = 0, z/D = 0.002 (refer to table 1 for
numbering of the scalar fields and respective parameters). In
each panel of the figure, a curve represents the ensemble average of the data on equispaced discrete bins of the horizontal
axis. In addition, the curve is color-coded according to theprobability of occurrence of data within a certain discrete bin.
The pair (φ2 , φ3 ) represents a reference case since boththe scalars are characterized by same Peclet number and wallboundary conditions. For such a combination, the linear behaviour depicted in figure 8(a) is expected. Indeed, despite thedifferent inflow bounary conditions, the two fields are characterized by exactly the same mean distribution. Deviations fromthe straight line of the reference case are observed whenever a

z/D

(a)
2
1.5
1
0.5
0
−4

1
0.8
0.6
0.4
0.2
0
−3

−2

−1

0

1

2

3

4

x/D

z/D

(b)
2
1.5
1
0.5
0
−4

1
0.8
0.6
0.4
0.2
0
−3

−2

−1

0

1

2

3

4

x/D

Figure 7. Time averaged distribution of passive scalar fields
in the plane y/D = 0. (a) φ1 ; (b) φ3 .

change in either wall boundary conditions or Peclet number
occurs. For instance, the effect of decreasing the Peclet number for one of the two scalar fields can be pictured in figure8(b). In this case, by comparison with figure 8(a), a broader
scatter characterizes the scalar field φ5 for any given value of
φ2 . Note that φ5 differs from φ3 only because of a smaller
Peclet number; hence, the greater scatter of data observed must
be associated to the stronger diffusion that characterizes thescalar field φ5 with respect to the scalar φ3 . Nonetheless, averaging over the sampled data evidences a mean behavior very
similar to the one observed for the pair (φ2 , φ3 ).
Similarly, the effect of varying the wall boundary condi-

412th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
(a)
1

0.025

0.8

0.02

0.6

0.015

0.4

0.01

0.2

0.005

(a)
1

0

0.8

0

z/D

φ3

−0.05

0.2

0.4

0.6

0.8

−0.1
0.4
−0.15

0.2

0
0

0.6

0
−0.2

1

φ2

0

0.2

−0.2

x/D

(b)
1

0.025

0.8

0.02

0.6

0.015

0.4

0.01

0.2

0.005

(b)
1

0

0.8

z/D

φ5

−0.05

0

0.2

0.4

0.6

0.8

−0.1
0.4
−0.15

0.2

0

0

0.6

0
−0.2

1

φ2

0

0.2

−0.2

x/D

(c)
1

0.025

0.8

0.02

0.6

0.015

0.4

0.01

0.2

0.005

(c)
1

0

0.8

z/D

φ3

−0.05

0

0.2

0.4

0.6

0.8

−0.1
0.4
−0.15

0.2

0

0

0.6

0
−0.2

1

φ1

0

0.2

−0.2

x/D

Figure 9. Color maps of the parameter δ representing thedeviation from the “linear” behaviour displayed by the scatter plots at a given location in space. The panels represent
the scalar field pairs: (a) (φ2 , φ3 ); (b) (φ2 , φ5 ); (c) (φ1 , φ3 ).
Streamlines are also depicted by black lines to identify the location of the recirculating region.

Figure 8. Scatter plots of passive scalar fields at x/D =, sampled values. The mean
0.022, y/D = 0, z/D = 0.
over the ensemble of sampled values is depicted by a solid line
in each plot and it is color-coded according to the probability
associated to the realization of events of the scalar fields in theconsidered range of values. Error bars represent one standard
deviation interval.

8(c) can be interpreted.
At a given location, a quantitative measure of the departure from the linear relation occurring between two passive
scalars can be identified through the parameter δ , which is defined as:tion for one of the two considered fields can be envisaged by
the comparison of figures 8(a) and (c). Because of the zero
Dirichlet boundary condition, non-zero flux through the wall
can occur to the scalar φ1 at the wall, thus influencing the values of the scalar in the near-wall region. In the particular caseconsidered, an outward flux at the wall determines the occurrence of lower values of the field φ1 in the near-wall region
when compared to values of φ2 at the same locations. As a result, the deviation from the linear behaviour observed in figure

δ=

Z 1
0

[⟨φi (s)⟩ − (1 − s)] ds(4)

In the equation, ⟨·⟩ indicates the ensemble average operator.

5

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
Hence, δ represents the signed area between the straight line
φ = 1 − s and the curve of the ensemble averaged values of thedata on discrete bins.
Color maps of the distribution of δ in the impingement
region are reported in figure 9 for the same passive scalar pairsconsidered before. While for the pair (φ2 , φ3 ) δ is zero everywhere (figure 9(a)), regions where evident departure fromthe linear behaviour are easily identified for the scalar pairs(φ2 , φ5 ) and (φ1 , φ3 ) in figure 9(b) and (c) respectively. It is observed that the largest deviations from the reference linear behavior occur for the case of variation of wall boundary condition from zero flux to homogeneous Dirichlet. In this respect,
the color map of figure 9(c) carries the information about theregion of the flow that is directly affected by such a change ofwall boundary condition in one of the scalar fields.

2530.
Dairay, F., Fortuné, V., Lamballais, E. & Brizzi, L. E. 2015
Direct numerical simulation of a turbulent jet impinging ona heated wall. J. Fluid Mech. 764, 362–394.
Dairay, T., Fortuné, V., Lamballais, E. & Brizzi, L.E. 2014 Lesof a turbulent jet impinging on a heated wall using highorder numerical schemes. Int. J. of Heat and Fluid Flow 50,
177–187.
Dewan, A., Dutta, R. & Srinivasan, B. 2012 Recent trends
in computation of turbulent jet impingement heat transfer.
Heat Transfer Engineering 33 (4-5), 447–460.
Draksler, M., Končar, B. & Cizelj, L. 2019 On the accuracy oflarge eddy simulation of multiple impinging jets. International Journal of Heat and Mass Transfer 133, 596–605.
Fisher, P. F., Lottes, J. W. & Kerkemeier, S. G. 2008-2021
NEK5000 v19.0. http://nek5000.mcs.anl.gov/ Argonne National Laboratory, Illinois.
Geers, L.F.G. 2004 Multiple impinging jet arrays: an experimental study on flow and heat transfer. PhD thesis, Technische Universiteit Delft.
Geers, L.F.G., Hanjalić, K. & Tummers, M. J. 2006 Wall imprint of turbulent structures and heat transfer in multiple impinging jet arrays. J. of Fluid Mech. 546, 255–284.
Geers, L.F.G., Tummers, M.J., Bueninck, T.J. & Hanjalić, K.
2008 Heat transfer correlation for hexagonal and in-line arrays of impinging jets. Int. J. of Heat and Mass Transfer
51 (21), 5389–5399.
Geers, L.F.G., Tummers, M.J. & Hanjalić, K. 2001 PIV measurements in impinging jets at a high reynolds number.
In Proc. Second Int. Symposium on Turbulence and Shear
Flow Phenomena. (ed. E Lindborg et al.), pp. 289–295.
Geers, L.F.G., Tummers, M.J. & Hanjalić, K. 2004 Experimental investigation of impinging jet arrays. Experiments
in Fluids 36, 946–958.
Kakka, P. & Anupindi, K. 2021 Flow and thermal characteristics of three-dimensional turbulent wall jet. Physics of Fluids 33 (2), 025108.
Maday, Y. & Patera, A.T. 1989 Spectral element methods forthe incompressible Navier-Stokes equations. In State of theart surveys on computational mechanics ASME, pp. 71–
143.
Na, Y. & Hanratty, T.J. 2000 Limiting behavior of turbulent
scalar transport close to a wall. International Journal of
Heat and Mass Transfer 43 (10), 1749–1758.
Thielen, L., Jonker, H.J.J. & Hanjalić, K. 2003 Symmetry
breaking of flow and heat transfer in multiple impinging
jets. Int. J. of Heat and Fluid Flow 24 (4), 444–453.
Viskanta, R. 1993 Heat transfer to impinging isothermal gas
and flame jets. Experimental Thermal and Fluid Science
6 (2), 111–134.
Zuckerman, N. & Lior, N. 2005 Impingement Heat Transfer: Correlations and Numerical Modeling. Journal of Heat
Transfer 127 (5), 544–552.

CONCLUSIONSThis work presents the DNS of two parallel square turbulent jets impinging on a flat solid wall. The study considers an
incompressible flow of a Newtonian fluid with constant properties and in the absence of external volume forces. Several
passive scalar fields were simulated in order to investigate theeffect that wall boundary conditions and diffusivity parameter
have on the global mean distribution of the scalar fields.
The study shows that the examined flow configuration
produces a very complex mean flow field which includes twolarge counter-rotating vortical structures in between the twojets in the impingement region. The presence of such structures has profound effects on the overall structure of the mean
flow field and, ultimately, on the mean distribution of the passive scalar fields. Deviations from a reference distribution of
two scalar fields shows the effects of varying diffusivity properties and wall boundary conditions in one of the consideredscalar fields. The largest deviations are associated to a variation of wall boundary conditions and the effects of such achange are felt up to z/D ≈ 0.1. In particular, this extended
region of influence of wall boundary conditions is linked tothe continuous upward fluid motion generated by the presenceof the two counter-rotating vortical structures between the two
jets.

ACKNOWLEDGEMENTS
We gratefully acknowledge the financial support of
the Deutsche Forschungsgemeinschaft through project
237267381-TRR150. Computations were performed at theHigh Performance Computing Center Stuttgart (HLRS) under
the grant number zzz44198..

REFERENCES
Barata, J.M.M. 1996 Fountain flows produced by multiple impinging jets in a crossflow. AIAA Journal 34 (12), 2523–

6