Cfd Report.docx

CFD Modelling of Supersonic Airflow Generated by 2D Nozzle Paper by:

SKANDA HEBBAR H S (4NI15ME156) and VINOD (4NI15ME184).

Introduction: A nozzle is a device which is used to give the direction to the gases coming out of the combustion chamber. Nozzle is a tube which has a capacity to convert the thermo-chemical energy generated in the combustion chamber into kinetic energy. The nozzle converts the low velocity, high pressure, high temperature gas in the combustion chamber into high velocity gas of lower pressure and low temperature. A convergent divergent nozzle is used if the nozzle pressure ratio is high. High performance engines in supersonic aircrafts generally incorporate some form of a convergent-divergent nozzle. Our analysis is carried using software’s like Ansys Workbench for designing of the nozzle and Fluent 15.0 for analysing the flows in the nozzle. In the present days there is a huge development in Aerospace Engineering for in various prospects. Extensive research is being carried out in the fields like civil and defence prospects. The virtualization is one of the major developments in the field of research, which has revolutionized Aerospace engineering, along with all other branches. The computational techniques are being used widely for getting better results, which are close to experimental techniques. The flow through a convergent-divergent nozzle is one of the benchmark problems used for modelling the compressible flow through computational fluid dynamics. In this paper CFD analysis of a convergent divergent rocket nozzle is done by varying the number of divisions in Mesh and obtaining results for various parameters like pressure, temperature, properties, wall fluxes, Mesh, velocity and adaption.

AIM: The aim of the paper is to find the exit velocity and the amount of pressure at the exit of the nozzle of a rocket by the initial parameters from the paper given below and also to check the velocity pattern at the exit if an obstacle is present and thus to find the nature of deviation of the velocity.

Analysis and Result: The nozzle used for the analysis is as shown below. The Initial conditions assumed are, 1. Pressure in vacuum tank for all tests was of the order of 5 mbar, while inlet values slightly varied depending on ambient conditions. 2. Atmospheric pressure 1018.313 mbar 3. Atmospheric temperature 286.75 K. 4. Reference Mach number 0.086, achieved in test installation in front of the nozzle, 5. Total pressure in wind tunnel test section 1010.542 mbar The Dimensions of the model taken,

This Model is replicated in solid works and then is imported to Ansys and further Modifications are done to set the need of the meshing. The model looks as,

This model is then taken into Ansys Meshing into order to subdivide the entire model into multiple node and elements. The meshing is as shown.

There are 15623 elements in the given 2D model. The Aspect ratio and Jacobean quality is found to be near 1 and skeeweness is near zero. The elements used are quadratic elements, thus the result is almost accurate. The number of iterations carried out are 4500 giving higher accuracy. The conditions and the parameters considered are, Calculation of flow characteristics inside the adopted control volume were performed using RANS (Reynolds-Averaged Naiver-Stokes) equations with k-omega SST (Shear-Stress Transport) turbulent model [11], [12], [13]. 

Solver: 2D density-based.

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Model: viscous, SST k-omega with compressibility effects.

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Fluid: air, ideal gas, viscosity by Sutherland law, three coefficient methods.

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Boundary conditions: control volume inlet and outlet parameters as defined in [1], for given test case.

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Calculation: flow type – supersonic, FMG - the Full Multi-Grid solution initialization at 4 levels [13], [14], initial optimum reordering of the mesh domain using Reverse Cut hill-McKee method [14], active solution steering, applying automatic optimization of Courant number for the achieved solution convergence stage, etc.

Thus, the result obtained are as follows,

Fig 3: shows the distribution of the velocity pattern along the Rocket Nozzle

Fig 4: shows the distribution of the Pressure pattern along the Rocket Nozzle

It can be seen that the Pressure is high at the inlet and velocity of exit is greater at the exit. The validation of the result is carried out in the next chapter.

Validation: This experiment is carried out trying to replicate the paper shown below. The Initial Conditions taken are almost same, although the entire parameters can’t be obtained, it can be

noted that the ratios of the two results must be almost same. The results obtained in the paper are as shown below,

By comparing the above given figures with the Fig 3 and Fig 4, it can be seen that the exit velocity in Fig 6 is 4.25E3 and in the Fig 3 is 1.3E3. Although there is a deviation in the result obtained, the pattern of the velocity deviation in both the cases is almost same. Thus, considering that the deviations is due the change is calculation procedure, meshing accuracy and change in initial conditions it can be said that both the results match with each other and the result is Validated.

New boundary Condition, Analysis and Result: After analysing the above Pattern of the nozzle exit, changes are done at the exit. An obstacle has been kept at the exit of the nozzle, and then the pressure and velocity variations over the flow is analysed. The effect of the obstacle and dimension and position of the obstacle is as shown below.

The obstacle is introduced soon after the nozzle exit and at the bottom. It is 15mm in length and 20mm in width. The obstacle is considered to infective to the high temperature and is a rigid body and hence doesn’t undergo deformation due to heat also doesn’t displace due o high velocity. The same previous conditions are assumed and the analysis is carried. In order to accommodate proper meshing of the obstacle, the model is further divided into multiple faces of polygon and then meshing is done. Care is take to have quad element ratio, Jacobian ratio and element quality ratio near one and skweeness ratio to near zero. The results of the analysis is as shown in the figure.

Fig 7: shows the distribution of the velocity pattern along the Rocket Nozzle

Fig 8: shows the distribution of the Pressure pattern along the Rocket Nozzle

Conclusion: The difference in the flow pattern is clearly visible. From this we can infer that if there is a flow deviation due to obstacle, the resultant force also deviates causing undesired motion. In the above scenario, the obstacle had caused the flow to be deviated to right causing unbalanced forced to the left. Under this case the Rocket wouldn’t be able to maintain stability and would move left of the centre. This representation also helps us to know the behaviour of flow when encountered with unknown object.

References: 1. REFERENCE PAPER: CFD Modeling of Supersonic Airflow Generated by 2D Nozzle by Olivera P. Kostić Research Assistant University of Belgrade Faculty of Mechanical Engineering Innovation Centre 2. Stefanović, Z., Miloš, M., Todić, I., Pavlović, M.: Investigation of the Pressure Distribution in a 2D Rocket Nozzle with a Mechanical System for Thrust Vector Control (TVC), Strojarstvo, Vol. 53 (4), pp 287-292, 2011. 3. Jojić, B., Milinović, M., Stefanović, Z., Blagojević, D.: Pressure Distribution in Rocket Nozzle with Mechanical System for TVC, AIAA Paper 87- 1824, June 1987. 4. Nauparac, D., Prsić, D., Miloš, M., Samadžić, M.Isaković, J.: Design Criterion to Select Adequate Control Algorithm for Electro-Hydraulic Actuator Applied to Rocket Engine Flexible Nozzle Thrust Vector Control Under Specific Load, FMETransactions, Vol. 41, No. 1, pp. 33-40, 2013.