**3. Glimpse of flowfield over reentry vehicles**

The flowfield features over the reentry capsule can be delineated through the experimental and theoretical investigations at high speed. The nomenclature and geometrical parameters of a typical reentry capsule is depicted in **Figure 1(a)** that leads to the necessity to investigate the influence of the geometrical parameters such as *D*, *αN*, *RC*, *αB*, and *L* on the flowfield and aerodynamic characteristics. A schematic sketch of flowfield is delineated in **Figure 1(b)** based on shadowgraph and schlieren pictures. The significant flow features are described by the following. In the fore-body section of the capsule, the fluid decelerates through the bow shock wave depending on the cruise speed and altitude. At the shoulder of the capsule, the flow turns and expands rapidly and boundary layer detached, forming a free-shear layer in the back-shell region that separates the inner recirculating flow region behind the module from the outer inviscid flowfield. The latter is recompressed and turned back to the freestream direction, first by the so-called lip shock wave, and further downstream by the recompression shock wave. At the end of the recirculating flow past the neck, the free-shear layer develops in the wake trail. A complex flow structure often includes a lip shock wave associated with the beveled expansion fan and wake trail adjacent to the shear layer confluence. The corner expansion process is an expansion fan pattern changed by the presence of the approaching boundary layer and radius of the bevel or shoulder, *RC*. The wake flow features

*Numerical Simulation of Base Pressure and Drag of Space Reentry Capsules at High Speed DOI: http://dx.doi.org/10.5772/intechopen.83651*

**Figure 1.** *Representation (a) geometrical parameters (b) flow features.*

*Hypersonic Vehicles - Past, Present and Future Developments*

The base pressure coefficient can be expressed as

*CPB* <sup>=</sup> \_\_\_\_\_\_\_ <sup>−</sup>*p*<sup>∞</sup>

reentry space capsule must satisfy inequality

CDB < \_\_\_\_\_ <sup>2</sup>

**3. Glimpse of flowfield over reentry vehicles**

mentally by Suzuki, and Abe [51].

blunt-cone/flare delft aerospace recovery test (DART) configuration is numerically analyzed by Otten [48] solving a laminar Navier-Stokes equations. Numerical analysis over blunted cone flare has been carried out at Mach 6 by Savino et al. [49]. Barnhardt [50] has carried out numerical simulation of flowfield in the wake region of a reentry vehicle at high speeds. The EXPeriment and Recovery of Space System (EXPRESS) reentry capsule at transonic and supersonic speeds is studied experi-

It is important to state here that the base pressure can never be less than zero.

Lower pressure is acting on the base experiences another form of aerodynamic base drag. The base drag coefficient based on the maximum cross-section of the

γM∞

Thus, it can be noticed that the base pressure is having complex flow features which are a function of several variables such as geometrical parameters of the foreand after-body of the reentry space vehicle, Mach number and Reynolds number. The measurements of base pressure in the wind-tunnel testing are affected by the presence of the sting attachment to the model. The free-flight experiment needs pneumatic launcher mechanism, pressure transducer, motion picture photography equipment, antenna, receiver and recording devices. However, the base pressure data obtained from the free-flight experiments are not affected by the sting attachment to the model as in the wind-tunnel testing. The numerical simulations are most suitable and inexpensive tool to evaluate flow characteristics, base pressure and drag coefficient for wide range of Mach numbers and Reynolds numbers.

The flowfield features over the reentry capsule can be delineated through the experimental and theoretical investigations at high speed. The nomenclature and geometrical parameters of a typical reentry capsule is depicted in **Figure 1(a)** that leads to the necessity to investigate the influence of the geometrical parameters such as *D*, *αN*, *RC*, *αB*, and *L* on the flowfield and aerodynamic characteristics. A schematic sketch of flowfield is delineated in **Figure 1(b)** based on shadowgraph and schlieren pictures. The significant flow features are described by the following. In the fore-body section of the capsule, the fluid decelerates through the bow shock wave depending on the cruise speed and altitude. At the shoulder of the capsule, the flow turns and expands rapidly and boundary layer detached, forming a free-shear layer in the back-shell region that separates the inner recirculating flow region behind the module from the outer inviscid flowfield. The latter is recompressed and turned back to the freestream direction, first by the so-called lip shock wave, and further downstream by the recompression shock wave. At the end of the recirculating flow past the neck, the free-shear layer develops in the wake trail. A complex flow structure often includes a lip shock wave associated with the beveled expansion fan and wake trail adjacent to the shear layer confluence. The corner expansion process is an expansion fan pattern changed by the presence of the approaching boundary layer and radius of the bevel or shoulder, *RC*. The wake flow features

<sup>2</sup> (1 − \_\_\_ *pB*

*<sup>p</sup>*∞) (1)

<sup>2</sup> (2)

\_1 <sup>2</sup> *<sup>ρ</sup><sup>∞</sup> <sup>V</sup>*<sup>∞</sup>

**118**

**Figure 2.** *Illustrations of flowfield over (a) cone (b) space vehicle at M∞ = 3.1.*

show several flowfield features such as free-shear layers, contraction of flow (neck) region and recompression shocks. The base flowfields also exhibit near and far wake region as depicted in **Figure 1(b)**. The values of *Lc* and *h* as depicted in **Figure 1(a)** are function several flow variables as mentioned above. The base plane of the capsule experiences another stagnation point.

**Figure 2(a)** and **(b)** has been drawn with the help of shadowgraph pictures of a 12.5° semi-cone and a blunt body capsule at high speed. The base pressure profile is illustrated in the wake region of the space vehicles. The schematic sketches as shown in **Figure 2** delineate a complex flowfield features associated with the nonlinear base pressure variations in the wake region.
