Demonstration of skin friction measurements featuring in situ estimation of conduction loss using constant voltage anemometers and surface hot-films
Authors: Sarma Garimella R. , Moes Timothy R. 
Published: April 2005

The top of the 12.2m long NASA Dryden Flight Research Center’s ground research vehicle (GRV) was used as a flat plate test bed for demonstrating an approach to measure skin friction. Using an array of surface hot-films operated by constant voltage anemometers (CVAs), the approach was demonstrated with in situ estimation of conduction heat loss from the hot-films to the substrate. An algebraic relationship, using the channel calibration constants a and b (determined a priori) with CVA output voltages Vs and Vw from that channel, is used for the estimation of the required quantities and lead resistance (rL) of the hot-film measured on site. Estimates of the power dissipated in the hot-film alone (Phf) (excluding the lead resistances), in situ resistance (Rw) of the hot-film due to applied overheat and flow, and the cold resistance (Ra) of the same hot-film at the ambient temperature are so obtained. Different approaches to estimate the in situ cold resistance (which is the resistance without any self-heating) of the hot-film are presented addressing the suitability of the procedure for flight applications as well. Tests were performed at several speeds of the GRV on the tarmac of a runway at the flight test center. The measured values are fitted to the classical (13) law equation with the computational dimensional skin friction (τ) obtained using the empirical local skin friction law for the long flat plate. There was an excellent (1∕3) law fit in all the hot-films, demonstrating that the measured values fit classical theory. Using this measured fit with the theoretical values, calibration coefficients (A and B) for dimensional skin friction (τ) were obtained. Using these calibration coefficients, measured values were then converted to nondimensional local skin friction coefficients cf for all the hot-films at all speeds. Measured cf values agree well with the associated flat plate theory. Since the in situ measurement of heat loss to the substrate should ideally apply at all ambient temperatures, the method was tested in a wind tunnel at two ambient temperatures, where the results repeated well. Accounting for conduction loss had been the difficult issue in using hot-films, which otherwise offer many advantages. For example, skin friction from an array of hot-films can be measured simultaneously and the hot-films offer large dynamic response, high sensitivity, and simple installation. In practice, it should be possible to insert a calibrated plug in the test article and use the in situ measured substrate loss to accurately estimate the skin friction. The approach does not need any additional temperature sensor to accomplish the measurements. The system is highly amenable to automation with further work. It may be noted that the purpose of this test and article is to demonstrate an approach to skin friction measurements using hot-films operated by CVAs with in situ estimation of the heat loss to the substrate. It does not address any other aspect of the skin friction measurements.