Induced Body Force Research Articles

Preswirl Maneuvering Propulsor

Recent concept studies have demonstrated the potential to utilize a preswirl propulsor configuration with adjustable upstream stator blades to generate propulsor side forces. These studies led to a set of experiments and corresponding computations to validate this concept. Ducted and open preswirl propulsors were configured with an upstream stator row and downstream rotor. During normal operation, the upstream stator blades are all situated at the same pitch angle and preswirl the flow into the propulsor while generating a roll moment to counter the moment produced by the rotor. By varying the pitch angles of the stator blade about the circumference, it is possible to both generate a mean stator side force and subsequently vary the axial velocity and swirl that is ingested into the rotor. The rotor then generates side forces in response to the modified inflow. Wind tunnel experiments were conducted to measure the steady, spatially varying stator wake flows for various stator geometric configurations using stereoscopic particle image velocimetry. Water tunnel experiments were conducted to measure the forces produced. Experimental data were used to validate computations, which utilized fully viscous 3-D [Reynolds averaged Navier-Stokes (RANS)] computations to predict the stator forces, velocity field, and rotor response. Both methods provided insight into the underlying mechanisms of side force generation. Optimized rotor designs were specifically investigated to isolate the blade forces as well as the induced body forces. In this way, RANS provided high fidelity performance predictions of the final propulsor design. Experimental and computational data demonstrated that total side force coefficients on the order of 0.26 could be generated by the open propulsor. This amount of control authority exceeds current control surface capabilities for Navy 21 (0.5334 m) unmanned undersea vehicles and is comparable to novel thrust-vectored designs.

Reduction of Fan and Compressor Wake Defect Using Plasma Actuation for Tonal Noise Reduction

This paper proposes a new technique to reduce the noise generated by rotor-stator interaction (tonal noise) in fans and compressors. The method involves the use of single dielectric barrier discharge (plasma) actuators near the blade trailing edge to reduce blade wakes. Plasma actuators are a new and simple type of active flow control device consisting of two parallel and offset electrodes separated by a layer of dielectric material. The application of a high ac voltage at high frequency to the electrodes generates a body force on the flow in the vicinity of the electrodes to inject momentum without mass addition. A preliminary assessment of the proposed concept is performed with a computational study on modern low-speed compressor rotor geometry. A plasma actuator model is implemented in an established turbomachinery CFD code. Simulations are carried out to evaluate the effect of the actuator strength, location, and actuation method (continuous versus pulsed) on the rotor wake. Results show that plasma actuators operated in continuous mode near the trailing edge can significantly influence the wake of the rotor with relatively little power consumption. The effectiveness of the actuation is proportional to actuator strength (induced body force). The exact position of the actuator in the trailing edge region has little effect on the effectiveness of the actuation. The results from simulations with pulsed actuation show very low time-averaged influence on the wake and are not fully conclusive, due possibly to the frequencies simulated and the limitations of the RANS CFD tool.

The rich structure of transition in a shear flow

Significant advances have been made in applying ideas from nonlinear dynamical systems theory to flows which exhibit sequences of bifurcations in the transition to turbulence. Moreover, the recent discoveries of finite-amplitude states in linearly stable flows holds great promise for a breakthrough in our understanding transition in shear flows. Tsukahara, Tillmark & Alfredsson (J. Fluid Mech., 2010, this issue, vol. 648, pp. 5–33) study a novel variant of a classical shear flow by adding global rotation. The competition between the induced body force and shear-induced instabilities leads to the discovery of a rich and beautiful tapestry of transition sequences.

Generation of large‐scale gravity waves and neutral winds in the thermosphere from the dissipation of convectively generated gravity waves

We study the response of the thermosphere and ionosphere to the dissipation of gravity waves (GWs) excited by a deep convective plume on 1 October 2005 at 52.5°W, 15.0°S, and 2120 UT in Brazil. Those small‐ and medium‐scale GWs which reach the thermosphere dissipate at z ∼ 120–250 km in a direction opposite to the background wind ∼(1–2) density scale heights below. This localized momentum deposition creates horizontal thermospheric body forces that have large sizes and amplitudes and generates large‐scale secondary GWs and large‐scale traveling ionospheric disturbances (LSTIDs) that propagate globally away from the body force in all directions except that perpendicular to the force direction. For the convective plume at 2120 UT, the secondary GWs have horizontal wavelengths of λH ∼ 2100–2200 km, periods of τr ∼ 80 min, horizontal phase speeds of cH ∼ 480–510 m/s, density perturbations as large as ∣ρ′/∣ ∼ 3.6–5% at z = 400 km, relative [O] perturbations as large as ∼2–2.5% at z = 300 km, and total electron content perturbations as large as ∼8%. This transfer of momentum from local, relatively slow, small scales at the tropopause to global, fast, large scales in the thermosphere is independent of geomagnetic conditions. The various characteristics of these large‐scale waves may explain observations of LSTIDs at magnetically quiet times. We also find that this body force creates a localized “mean” horizontal wind in the direction of the body force. For the plume at 2120 UT, the wind is southward with an estimated maximum of vmax ∼ −400 m s−1 that is dissipated after ∼4 h. We also find that the induced body force direction varies throughout the day, depending on the winds in the lower thermosphere.

Hyperacceleration effects on turbulent combustion in premixed step-stabilized flames

Experimental results are presented from an investigation of the effects of large transverse accelerations on flame propagation and blowout limits in premixed step-stabilized flames. The accelerations, which exceed ±10,000 g in the present study, induce large body forces on the high-density reactants and low-density products. These body forces can substantially alter the flame propagation mechanisms and dramatically increase the flame blowout limits. Sustained centripetal accelerations a c ≡ U 2/ R are created by flowing a premixed propane–air reactant stream with equivalence ratios 0.7 ⩽ Φ ⩽ 1.9 at various speeds U through a semicircular channel with radius R. A backward-facing step of height h on the radially outer ( a c > 0) or inner ( a c < 0) wall stabilizes the flame. For a c > 0 the acceleration acts to force high-density reactants into the recirculation zone and low-density products into the reactant stream, while a c < 0 forces hot products into the recirculation zone and impedes cold reactants from entering this zone. An otherwise identical straight channel provides corresponding baseline ( a c = 0) results for comparison. The flow speed U, equivalence ratio Φ, and step height h are systematically varied for a c = 0, a c > 0, and a c < 0. Shadowgraph and chemiluminescence imaging show that as a c→ +∞ the propagation of the flame across the channel becomes independent of the flame burning velocity and instead is primarily due to large-scale “centrifugal pumping” driven by the induced body forces. For a c → −∞ the body forces effectively segregate reactants and products to produce a nearly flat flame. In both cases, for large | a c| values the resulting blowout limits can be substantially higher than those at a c = 0.

A study of the characteristics of slotted laminar jet diffusion flames in the presence of non-uniform magnetic fields

The behavior of laminar jet diffusion flames in the presence of non-uniform magnetic fields has been investigated and the results of this experimental study are presented. It has long been recognized that magnetic fields can influence the behavior of laminar diffusion flames as a result of the paramagnetic and diamagnetic properties of the constituent gases. Using a magnet assembly consisting of neodymium iron boron magnets and gray steel prisms, a non-uniform upward decreasing magnetic field was applied to a laminar jet diffusion flame produced using a slotted burner port. The experimental results show that under certain conditions the application of the magnetic field decreased the flame height, prevented the flame from attaching to the prisms, increased the intensity of the flame, decreased the flow rate for which visible soot inception occurred, and increased the flow rate below which the flame extinguished. It was also observed that the degree to which these phenomena occur is proportional to the product of the magnetic induction times the gradient of the magnetic induction. A discussion as to the reasons for these observations is provided. In addition to traditionally defined dimensionless parameters, the previously defined magnetic Grashof number, the magnetic Froude number, and the ratio of the gravitationally induced buoyancy forces to the magnetically induced body forces are identified as key parameters for determining when a magnetic field will affect diffusion flame behavior. Using these dimensionless parameters, the experimental data was cast in a form that clearly indicated the universal nature of diffusion flame behavior in a non-uniform upward decreasing magnetic field. A power law curve fit to the dimensionless data is presented and discussed.

Suppression of the Vortex in Ladle by Static Magnetic Field.

The vortex formation in draining the melt in ladle can be suppressed by the reduction of the tangential flow velocity of the melt. Two magnetic devices were designed in order to reduce the tangential flow velocity and tested with wood metal melt. One consists of 4 permanent magnets (PM device) and the other consists of an electromagnet (EM device). Magnetic flux density around each device was calculated and compared with the measured one. Induced body force in the melt was calculated with both of the calculated magnetic flux density and velocity profile. The dimensionless vortex formation height, where the vortex formation height is divided by orifice inner diameter, decreased from 1.7 down to 0.85 in both cases of the PM device and the EM device as the static magnetic field increased up to 0.17 T.

Gravity wave forcing in the middle atmosphere due to reduced ozone heating during a solar eclipse

We present an analysis of the gravity wave structure and the associated forcing of the middle atmosphere induced by the screening of the ozone layer from solar heating during a solar eclipse. Fourier integral techniques and numerical evaluation of the integral solutions were used to assess the wave field structure and to compute the gravity wave forcing of the atmosphere at greater heights. Our solutions reveal dominant periods of a few hours, characteristic horizontal and vertical scales of ∼5000 to 10,000 km and 200 km, respectively, and an integrated momentum flux in the direction of eclipse motion of ∼5.6 × 108 N at each height above the forcing level. These results suggest that responses to solar eclipses may be difficult to detect above background gravity wave and tidal fluctuations until well into the thermosphere. Conversely, the induced body forces may penetrate to considerable heights because of the large wave scales and will have significant effects at levels where the wave field is dissipated.

Pore pressure influence on tensile fracture propagation in sedimentary rock

Theoretical analysis and experimental results are presented to describe the influence of pore pressure on tensile fracture initiation and propagation direction. Fracture is influenced by both pore pressure magnitude on a local scale around the crack tip and by the orientation and distribution of pore pressure gradients on a global scale. Simple fracture initiation tests conducted on cylindrical limestone and sandstone rock samples are first described which support an effective stress law for tensile fracture initiation in very porous rocks. In other experiments, controlled fractures are propagated both mechanically and hydraulically across sandstone slabs subjected to non-uniform pore pressure fields. The fractures are shown to propagate towards regions of higher local pore pressure. Finallt, the influence of pressure gradient induced body forces is demonstrated by generating fractures connecting two pore pressure source points. The experimental results are supported by poroelasticity and fracture mechanics analysis of the process, and may be applied to influence hydrailic fracture orientation in the field.

Gravitational effects on chemically reacting laminar boundary layer flows over a horizontal flat plate

This is a theoretical study of the chemically reacting laminar boundary layer flow over a horizontal flat plate with gravitationally induced buoyant force. A diffusion flame sheet model was invoked to describe the combustion process. It was found that the effects of gravity on the purely force convection flow are characterized by a dimensionless coordinate quantity Gr x /Re x 5/2 . The governing equations were obtained by expanding the dependent variables into series in terms of this coordinate quantity. The problem inherently is a coordinate perturbation and it is treated as such. A numerical solution of the zeroth and first order governing equations subject to the appropriate physical boundary conditions was obtained for various external flow conditions such as wall temperature, freestream oxygen concentration and aiding and opposing flows. It was shown that the cross stream buoyancy induced body force acts effectively to produce a streamwise pressure gradient in the fluid adjacent to the plate surface. The pressure gradient is favorable in aiding flows (plate facing upwards) and is adverse in opposing flows (plate facing downwards). Hence, the local boundary layer flow is accelerated or decelerated relative to the corresponding gravity-free forced convection flow. Correspondingly, there is an increase or decrease in the local skin friction and heat transfer rates, and consequently a decrease or increase in the flame “stand-off” distance, depending upon whether aiding or opposing flow occurs, respectively. Based on the present study we conclude that buoyancy plays an important role in boundary layer diffusion flames, and by retaining the buoyant force the current model is capable of explaining phenomena that have been observed experimentally but have not been predicted by classical boundary layer theory. Such previously unaccounted for effects include the acceleration of the boundary layer flow (sometimes a velocity overshoot is exhibited), and a decrease in the flame “stand-off” distance.