Influence of strong perturbations on wall-bounded flows

Single-point hot-wire measurements are made downstream of a series of spanwise repeating obstacles that are used to generate an artificially thick turbulent boundary layer. The measurements are made in the near field, in which the turbulent boundary layer is beginning to develop from the wall-bounded wakes of the obstacles. The recent paper of Rodríguez-López et al. [E. Rodríguez-López et al., Phys. Rev. Fluids 1, 074401 (2016)] broadly categorized the mechanisms by which canonical turbulent boundary layers eventually develop from wall-bounded wakes into two distinct mechanisms, the wall-driven and wake-driven mechanisms. In the present work we attempt to identify the geometric parameters of tripping arrays that trigger these two mechanisms by examining the spectra of the streamwise velocity fluctuations and the intermittent outer region of the flow. Using a definition reliant upon the magnitude of the velocity fluctuations, an intermittency function is devised that can discriminate between turbulent and nonturbulent flow. These results are presented along with the spectra in order to try to ascertain which aspects of a trip's geometry are more likely to favor the wall-driven or wake-driven mechanism. The geometrical aspects of the trips tested are the aspect ratio, the total blockage, and the blockage at the wall. The results indicate that the presence, or not, of perforations is the most significant factor in affecting the flow downstream. The bleed of fluid through the perforations reenergizes the mean recirculation and leads to a narrower intermittent region with a more regular turbulent-nonturbulent interface. The near-wall turbulent motions are found to recover quickly downstream of all of the trips with a wall blockage of 50%, but a clear influence of the outer fluctuations, generated by the tip vortices of the trips, is observed in the near-wall region for the high total blockage trips. The trip with 100% wall blockage is found to modify the nature of the inner-wall peak of turbulent kinetic energy

Application of transpiration cooling on hypersonic vehicles to mitigate material oxidation

This thesis investigates the application of transpiration cooling to mitigate material oxidation of hypersonic vehicles. This was motivated by the early onset of surface oxidation of many high-temperature aerospace materials and the resulting limitation of the flight envelope. For example, Ultra-High-Temperature Ceramics have melting points exceeding 3500 K, thus could balance the incoming heat flux through re-radiation alone, but start oxidising at 1000 K, which limits the passive cooling capability severely. The aim of this thesis is to increase this oxidation limit by shielding the surface from freestream oxygen through boundary layer mass injection. A novel pressure sensitive paint diagnostic is developed in this work to measure the concentration of molecular oxygen on a transpiration cooled surface. This is a fundamental tool to quantify the ability of the coolant film to act as a barrier against freestream species that would lead to oxidation. The paint consists of [Ru(dpp)3]2+ luminophores, which are dissolved in a dichloromethane solution and then sprayed onto porous alumina. This method is validated experimentally on a transpiration cooled flat plate in a hypersonic cross-flow in the Oxford High Density Tunnel. Tests were conducted with no coolant injection, nitrogen injection and air injection at increasing blowing ratios. Oxygen partial pressure maps on the transpiration cooled surface were obtained for several conditions at unit Reynolds numbers between 2.58−5.0e7/m and blowing ratios between 0.016%−0.078%. It is found that the oxygen pressure decreases as the unit Reynolds number decreases and the blowing ratio increases. The direct measurements provided by this technique will aid the development of empirical correlations for the oxygen concentration on a porous surface with mass injection. Before applying this diagnostic to a transpiration cooled stagnation point, a theoretical prediction of the species concentration at the wall is derived. This is achieved by combining the self-similar boundary layer equations with thin film theory. The resulting semi-analytical correlation explicitly expresses the concentration of freestream species on a hypersonic stagnation point as a function of freestream conditions, surface geometry and coolant properties. The concentration depends on the boundary layer edge pressure, temperature and velocity gradient, as well as the wall temperature and injected mass flux. The molar mass and diffusion coefficient are further needed for scaling if the injected gas differs from the freestream gas. The method is compared against the numerically obtained self-similar solutions of a wide range of flow conditions and showed an accuracy of ±4%. The correlation was compared to experiments on a transpiration cooled stagnation probe model tested in the Oxford High Density Tunnel. A porous Al2O3 material, developed in collaboration with Imperial College London, features a similar pore size and outflow homogeneity as ZrB2, with the additional capability of bonding PSP. Experiments were conducted at Mach 6.9 at three different Pitot pressures: 10 kPa, 20 kPa and 30 kPa. Nitrogen, Argon and Krypton are used as injection gases at mass flow rates ranging from 0.01 - 0.55 kg/m2s, in order to displace up to 99% of the freestream gas at the surface. The experimental data shows that transpiration cooling is more effective in displacing freestream gas than predicted by analytical models and numerical solutions. The microheterogeneous surface with recessed pores means there is an additional pressure gradient within the first layer of pores, which increases the displacement effectiveness. A test campaign in the plasma wind tunnel at the Institute of Space Systems in Stuttgart provided the first qualitative data of the oxidation behaviour of transpiration cooled ZrB2 in a continuous, steady state high-enthalpy environment. A 42% porous ZrB2 sample is exposed to a cold-wall, fully catalytic stagnation point heat flux of 3.95 MW/m2 at 3.22 kPa Pitot pressure. While the uncooled sample fully oxidised at a surface temperature of 2150 K, 20.25 g/m2s of helium and 620.11 g/m2s of nitrogen injection mitigated oxidation of the transpiration cooled samples. Combining theoretical, numerical and experimental findings, the oxidation response of the stagnation point in a representative hypersonic flight scenario is predicted. An existing oxidation model for ZrB2 is combined with low order correlations for the heat transfer and oxygen concentration at the surface, to yield the surface recession and oxide scale thickness. A 500 s steady state trajectory at 44 km altitude and 3.6 km/s velocity is found to lead to a 2.2 mm recession of the 3 mm nose radius. A constant mass injection at a blowing parameter of 0.6 reduces the recession to just 0.21 mm. The displacement of freestream oxygen by transpiration cooling has a significant effect on oxidation. Not accounting for the displacement of oxygen at the surface would increase the recession by up to 197%. The recession along the transient trajectory of an envisioned hypersonic vehicle with a 3 mm nose radius is found to exceed 0.94 mm with no mass injection. It is shown that nitrogen and helium injection at a blowing parameter of 0.6 can reduce the recession to 0.13 mm and 0.075 mm, respectively.</p

Oxidation Response of Transpiration Cooled ZrB2 on a Hypersonic Stagnation Point - Dataset

This work presents the oxidation response of a transpiration cooled hypersonic stagnation point made of porous ZrB2. Low order models are used to calculate the surface temperature and oxygen concentration for a given flight condition. An analytical material oxidation model computes the surface recession and oxide layer thickness. A 500 s steady state trajectory at 44 km altitude and 3.6 km/s velocity is found to lead to 2.2 mm recession of the 3 mm nose radius. A constant mass injection at a blowing parameter of 0.6 reduces the recession to just 0.21 mm. The displacement of freestream oxygen by transpiration cooling has a significant effect on oxidation. Not accounting for the displacement of oxygen at the surface would increase the recession by up to 197%. The recession along the transient trajectory of an envisioned hypersonic vehicle with a 3 mm nose radius is found to exceed 0.94 mm with no mass injection. It is shown that nitrogen and helium injection at a blowing parameter of 0.6 can reduce the recession to 0.13 mm and 0.075 mm, respectively

Oxidation response of transpiration-cooled ZrB2 on a hypersonic stagnation point

This work presents the oxidation response of a transpiration-cooled hypersonic stagnation point made of porous ZrB2. Low-order models are used to calculate the surface temperature and oxygen concentration for a given flight condition. An analytical material oxidation model computes the surface recession and oxide layer thickness. A 500 s steady-state trajectory at 44 km altitude and 3.6  km/s velocity is found to lead to 2.2 mm recession of the 3 mm nose radius. A constant mass injection at a blowing parameter of 0.6 reduces the recession to just 0.21 mm. The displacement of freestream oxygen by transpiration cooling has a significant effect on oxidation. Not accounting for the displacement of oxygen at the surface would increase the recession by up to 197%. The recession along the transient trajectory of an envisioned hypersonic vehicle with a 3 mm nose radius is found to exceed 0.94 mm with no mass injection. It is shown that nitrogen and helium injection at a blowing parameter of 0.6 can reduce the recession to 0.13 and 0.075 mm, respectively

Correlation for species concentration on a hypersonic stagnation point with mass injection

This paper presents a correlation to obtain the surface species concentration on a stagnation point with mass injection for hypersonic flow. The correlation can be used to determine surface recombination heat fluxes and oxidation levels for ablatives or actively cooled heat shields in thermochemical equilibrium flow. It explicitly expresses the wall species concentration as a function of freestream conditions, coolant properties, and the stagnation-point geometry, revealing the direct effect of the aforementioned parameters on the desired quantity. The method was compared against the numerically obtained self-similar solution and showed an accuracy of ±4%. The concentration depends on the boundary-layer edge pressure, temperature, and velocity gradient, as well as the wall temperature and injected mass flux. The molar mass and diffusion coefficient are further needed for scaling if the injected gas differs from the freestream gas

Correlation for Species Concentration on a Hypersonic Stagnation Point with Mass Injection - Dataset

This paper presents a correlation to obtain the surface species concentration on a stagnation point with mass injection for hypersonic flow. The correlation can be used to determine surface recombination heat fluxes and oxidation levels for ablatives or actively cooled heat shields in thermochemical equilibrium flow. It explicitly expresses the wall species concentration as a function of freestream conditions, coolant properties, and the stagnation-point geometry, revealing the direct effect of the aforementioned parameters on the desired quantity. The method was compared against the numerically obtained self-similar solution and showed an accuracy of�4%. The concentration depends on the boundary-layer edge pressure, temperature, and velocity gradient, as well as the wall temperature and injected mass flux. The molar mass and diffusion coefficient are further needed for scaling if the injected gas differs from the freestream gas

Studying the film effectiveness of transpiration cooled walls using pressure sensitive paint

This paper presents the performance of pressure sensitive paint (PSP) for the direct investigation of a transpiration cooled surface. This technique allows the quantification of the pressure distribution on a porous surface with blowing. Additionally, it can be used to detect where the coolant is on the surface, thus measuring the transport of molecular oxygen to a transpiration cooled surface. For highly turbulent flows, it can also be used to evaluate the film effectiveness. A porous aluminium sample was anodised and dip coated in a PSP luminophore solution. It was fitted into a flat plate model and exposed to a Mach 5 cross-flow in the Oxford High Density Tunnel. Tests were conducted with no coolant injection, air injection and with nitrogen injection at increasing blowing ratios. The film effectiveness of the transpiration cooled surface was obtained for several conditions at Re = 15.5 - 31.5 10^6/m and F = 0.001 - 0.002. The film effectiveness increases as the Reynolds number decreases and the blowing ratio increases, which is in good qualitative agreement with the literature. Furthermore, it shows the same features as a velocity map of the outflow

Measuring the concentration of freestream species on a hypersonic transpiration-cooled stagnation point

This paper presents direct surface concentration measurements of a transpiration-cooled stagnation point in hypersonic flow. Pressure-sensitive paint is employed on a porous alumina sample to measure the concentration of freestream species and thus how well the coolant mitigates mass diffusion from the freestream to the surface. Experiments are conducted at Mach 6.9 at three different pitot pressures: 10, 20, and 30 kPa. Porous alumina is chosen due to its ability to bond pressure-sensitive paint and its similar microstructure to porous ultra-high-temperature ceramics. Nitrogen, argon, and krypton are used as injection gases at mass flow rates ranging from 0.01 to 0.55  kg/(m2⋅s), in order to displace up to 99% of the freestream gas at the surface. The experimental data show that transpiration cooling is more effective in displacing freestream gas than predicted by analytical models and numerical solutions. The microheterogeneous surface with recessed pores means that there is an additional pressure gradient within the first layer of pores

Measuring the concentration of freestream species on a hypersonic transpiration-cooled stagnation point - Dataset

This paper presents direct surface concentration measurements of a transpiration cooled stagnation point in hypersonic flow. Pressure sensitive paint (PSP) is employed on a porous alumina sample to measure the concentration of freestream species and thus how well the coolant mitigates mass diffusion from the freestream to the surface. Experiments are conducted at Mach 6.9 at three different Pitot pressures: 10 kPa, 20 kPa and 30 kPa. Porous alumina is chosen due to its ability to bond PSP and its similar microstructure to porous Ultra-High-Temperature Ceramics. Nitrogen, Argon and Krypton are used as injection gases at mass flow rates ranging from 0.01 - 0.55 kg/m2s, in order to displace up to 99% of the freestream gas at the surface. The experimental data shows that transpiration cooling is more effective in displacing freestream gas than predicted by analytical models and numerical solutions. The microheterogeneous surface with recessed pores means there is an additional pressure gradient within the first layer of pores

Studying the Film Effectiveness of Transpiration Cooled Walls Using Pressure Sensitive Paint

This paper presents the performance of pressure sensitive paint (PSP) for the direct investigation of a transpiration cooled surface. This technique allows the quantification of the pressure distribution on a porous surface with blowing. Additionally, it can be used to detect where the coolant is on the surface, thus measuring the transport of molecular oxygen to a transpiration cooled surface. For highly turbulent flows, it can also be used to evaluate the film effectiveness. A porous aluminium sample was anodised and dip coated in a PSP luminophore solution. It was fitted into a flat plate model and exposed to a Mach 5 cross-flow in the Oxford High Density Tunnel. Tests were conducted with no coolant injection, air injection and with nitrogen injection at increasing blowing ratios. The film effectiveness of the transpiration cooled surface was obtained for several conditions at Re = 15.5 - 31.5 10^6/m and F = 0.001 - 0.002. The film effectiveness increases as the Reynolds number decreases and the blowing ratio increases, which is in good qualitative agreement with the literature. Furthermore, it shows the same features as a velocity map of the outflow