Simcenter STAR-CCM+ How does it work: Re-Entry vehicles – Why a Blunt Body/nose?

INTRODUCTION

If we want to travel at high speeds in the atmosphere, the first thought is to have a good aerodynamic, what means a sharp pointed body. In the nature, for example, marlins and falcons have a pointed "nose".


Fig. 1: Pacific blue marlin. Credit: Ken Neill (https://www.fisheries.noaa.gov/species/pacific-blue-marlin)

To achieve supersonic/hypersonic flight, the airplanes have often pointed noses, e.g. Concorde, SR-71.


Fig. 2: Concorde - https://en.wikipedia.org/wiki/Concorde

Also, we can ask ourselves, why the re-entry vehicles are blunt bodies and the space shuttle has a blunt nose?

Fig. 3: Space Shuttle Columbia - (https://en.wikipedia.org/wiki/Space_Shuttle_Columbia)

To answer this question, we will try to set up a simulation using Simcenter STAR-CCM+ and see the shock waves created by the body, and where they interact.

SUPERSONIC/HYPERSONIC FLOW

But the first question is: what is a Supersonic/Hypersonic flow? To define a supersonic flow, we need to know that a pressure wave travels with so-called speed of sound in an elastic medium. If a body in this medium is travelling faster than the speed of sound, it is said, that the body is travelling supersonic. For velocities greater than 5 times the speed of sound, hypersonic. A characteristic of travelling supersonic is the shock waves around the body. The angle of the shock waves shows us how fast the body is travelling through the medium (See Fig. 4).


Fig. 4: doppler effect supersonic - https://en.wikipedia.org/wiki/Supersonic_speed

There are substantial technical challenges in supersonic conditions. As the aerodynamics of supersonic flight is dramatically different from those of subsonic flight (i.e., flight at speeds slower than that of sound). In particular, aerodynamic drag rises sharply as the aircraft passes the transonic regime, requiring much greater engine power and more streamlined airframes. [4]

So, why do the re-entry vehicles, like the space shuttle, have blunt bodies?

If we look at the temperature distribution on the re-entry, we will see that the gas is heated up after the shock wave, and the temperature is so high that we have a plasma around it. We want to have as few shock waves as possible and minimize the temperature.

Furthermore, the shock waves can interact with each other, and will bring more heat in the equation, what is not wanted. So, the design should have minimal inter-action between the shock waves with each other and the body of the vehicle.

After the shock waves, the flow is high temperature and turbulent, leading to more heat production and dispersion along the body, especially if they are near of the mainframe, which can cause material failure and/or melting, which for the shuttle would be catastrophal, because its mainframe is made of aluminium alloys. The main advantage of a blunt body, in this case, is that the shock waves are far from the mainframe, creating some kind of heat shield.

After the re-entry, the shuttle flies somehow like an airplane. The board-computers should be able to control it. If there are shock waves on the control surfaces, it is difficult to control the airplane. See the case of the first maned supersonic flight and the challenges involved, Fig. 5 (https://en.wikipedia.org/wiki/Bell_X-1 and https://www.flitetest.com/articles/bell-x-1-the-challenges-of-going-supersonic ).


Fig. 5: Bell X1 - (https://en.wikipedia.org/wiki/Bell_X-1)

The last point to consider, it is the reduction of the velocity in the re-entry. As the velocities are very high on the re-entry (> 5 km/s), and we would like to reduce the velocity as fast and controllable as possible, we need to have high drag geometry causing less shock waves, moving them far away from the surface. Hence, the only possible geometry is a blunt body.

In resume, why a blunt body for re-entry vehicles:

1. A “natural” heatshield.
2. A “natural” braking system.
3. Less inter-action between shockwaves and body.
4. Less heat dispersion.

Let us break the sound barrier in Simcenter STAR-CCM+

Some recommended Settings (see * for more recommendations and details)

1. Domain
   1. a sphere with a diameter of 8-10 Times the body length
   2. free Stream boundary condition
2. Physics
   1. coupled solver
   2. Advection Upstream Splitting Method (AUSM+FVS)
   3. Positivity Rate Limit < 0.05 in Coupled Solver
   4. Courant Number (CFL) < 0.1 and linear Ramp
3. Temperature:
   • < 800K: Ideal Gas
   • > 800K: Real Gas – Equilibrium Air

   Mach Number	Flow Regime	Modeling
   M < 3	Calorically Perfect	Ideal Gas/Constant Specific Heat
   3 < M < 8	Thermally Perfect	Ideal Gas/Specific Heat (polynomial in T or Table)
   8 < M < 30	Dissociation	Real gas model and/or chemically reacting flow
   M > 30	Ionization	Requires solving the Navier-Stokes equations together with the Maxwell equations. Not inherently supported in Simcenter STAR-CCM+.

4. Turbulent
   • K-Omega, Sparlat-Almaras, or sometimes Laminar
5. Mesh
   1. CAD Geometry is preferred
   2. Split Surfaces to control mesh refinements
   3. Fine Mesh to solve relevant geometry/physical features
   4. Coarse mesh in far field
   5. Smooth transition between prism layer and core mesh

Using some of these best practices, here are some results of the simulations of the shuttle “re-entering” into the atmosphere and a hypersonic flying human body to see the shock wave interaction

First, a comparison between the shock waves between a blunt body and a pointed body (the body used is a human body mock-up).

Fig. 6: Temperature Human Body - Blunt

Fig. 7: Temperature Human Body - Pointed

From the pictures above, we can see that the shock wave on the blunt body is far away from the body and it contains almost the complete body (Fig. 6). On the right picture, the shock waves are nearer on the body (Fig. 7) and with that, they have more inter-actions with the body and the other shock waves. In the Fig. 6 and 7, the shock waves can be considered the area where the color is yellow.

In the next simulation in Simcenter STAR-CCM+, we have tried to reproduce the Fig. 9, which is a wind tunnel test of the space shuttle. In the simulation, we have considered a Mach 10 velocity, a pressure of 10 kPa, and a fluid temperature of 200K. The Angle of Attack is 40 deg. In the Fig. 8, we can see that we have captured qualitatively all the main features shown in the Fig. 9. The shock waves look very similar on both pictures. 
 

Fig. 8: Re-Entry Space Shuttle Simulation

Fig. 9: Nasa Shuttle Test Using Electron Beam - From Wikipedia

At last, some fun:


Fig. 10: Rocket Scientists. Credits: The Farside by Gary Larson [7][9]


Sources

[1] The Insane Engineering of Re-Entry: https://www.youtube.com/watch?v=ivLX9o6Ayl8
[2] How Hypersonic Wind Tunnels Recreate Mach 20:https://www.youtube.com/watch?v=v0Z_4VyuzcA
[3] Engineering Connections (Richard Hammond) – Space Shuttle | Science Documentary | Reel Truth Science: https://www.youtube.com/watch?v=v1_WqVguRJg
[4] https://en.wikipedia.org/wiki/Supersonic_aircraft
[5] How to Land the Space Shuttle… from Space: https://www.youtube.com/watch?v=Jb4prVsXkZU
[6] https://en.wikipedia.org/wiki/Space_Shuttle
[7] https://www.thefarside.com/

SIEMENS
[8] https://blogs.sw.siemens.com/simcenter/mitigating-the-carbuncle-effect-for-hypersonic-cfd-simulations/
[9] https://blogs.sw.siemens.com/simcenter/10-insights-from-rocket-cfd-for-a-mind-blowing-moonshot/
[10] Visualizing Shocks Using Numerical Schlieren and Shadowgraph: https://support.sw.siemens.com/knowledge-base/KB000035684_EN_US
[11] How to create a solution-based mesh adaption methodology for capturing shocks: https://support.sw.siemens.com/knowledge-base/KB000039586_EN_US
[12] Modeling hypersonic cases in STAR-CCM+ (Mach > 10): https://support.sw.siemens.com/knowledge-base/KB000037988_EN_US
[13] Recommended Physics and Solver Setup for Aerospace External Aerodynamics Simulations: https://support.sw.siemens.com/knowledge-base/KB000053996_EN_US
[14] Carbuncle Mitigation in Hypersonic Athospheric Re-Entry with Apollo Capsule: https://support.sw.siemens.com/knowledge-base/KB000049034_EN_US
[15] Aerospace Vehicle External Aerodynamics Best Practices: https://support.sw.siemens.com/knowledge-base/KB000031728_EN_US
[16] User Guide: Transonic/Supersonic External Aerodynamics: Steady State RANS Approach
[17] Tutorial: Adaptive Mesh Refinement: Hypersonic Flow