## 1 Introduction  
Hybrid simulation can be defined as a variant of hardware-in-the-loop simulation where the tested object is a deformable mechanical structure. The method consists of performing numerical analysis and experiment at the same time. It applies to mechanical systems that have elements difficult or impossible to model numerically. These elements are in a hybrid simulation of the physical subsystem, are connected to actuators and deformation sensors, and tested in the experiment. The rest of the system is called the analytical subsystem and is modeled numerically using the finite element method (FEM). The main idea is that the physical subsystem is deformed during the simulation, exactly as it would be deformed as a part of the whole system under load. In order to achieve that, maintaining the full dynamical behavior of the system, all the finite element (FE) computations and data interchange between both subsystems must be performed in real-time [1,2]. The algorithm of hybrid simulation is presented in Figure 1[3]. The algorithm executes step-after-step. In every time step, \(i\) the displacement vector for the next time step \(\mathbf{u}_{i+1}\) is computed using FEM and imposed in the physical subsystem. The displacement develops forces in the physical subsystem, which are measured in the form of a vector \(\mathbf{r^{P}}_{i+1}\), where subsequent elements of the vector correspond to degrees of freedom (DOFs) of the discretized FE model [4; 5]. Many applications of hybrid simulation concern civil engineering and investigating the response of reinforced concrete or other structures to seismic excitations [6; 7; 8]. Mechanical systems containing nonlinear dumpers (e.g., magneto-rheological) are also typically investigated with the described method [9; 10; 11]. Hybrid simulation has also found broad applications in the automotive and aerospace industries [12; 13; 14]. Because FEM is a highly computationally demanding method, performing it in real-time may be difficult [15; 16; 17]. FE models of systems tested using hybrid simulations are rather simple, in most cases, built of the 1D beam, truss, or frame elements. Sources [6; 7; 8; 9] have described modern (2013-2016) results of hybrid simulations in civil engineering. The mechanical systems presented in the papers are frames and beams containing highly nonlinear elements (e.g., magnetorheological dampers) that must be tested experimentally. The problem of utilizing very complex FE models (e.g., 3D structure with thousands of DOFs) is that (a) performing FE computations in real-time with time steps of milliseconds or fractions of milliseconds would be highly difficult or impossible and (b) realization of experiment would be difficult as the number of actuators connected to every node of the interface between physical and analytical subsystem must be equal to the number of DOFs of this node. [PERSON] et al. [9] presented hybrid simulations of two structures with complex viscous dampers: wind turbine modeled with beam elements (eight DOFs) and shear frame with 10 DOFs. In the hybrid simulation, there is a conflict between accuracy, number of DOFs, and time step size. The more DOFs the tested system is discretized to, the greater the accuracy of the model is obtained;  
Figure 1: Hybrid simulation algorithm. however, the greater the time step must also be set. The greater the time step is set, the lower the accuracy of the simulation is obtained (taking into account the dynamic behavior of the structure). The current trend to solve the abovementioned problem, for systems that require discretization to relatively great numbers of DOFs, is to implement distributed computing, involving powerful hardware resources like computer clusters or supercomputers, where the communication between the laboratory with physical subsystem and computing resources is performed, utilizing peer-to-peer networks [18; 19; 20; 21].