Wind Tunnel Balance Research Articles

Research on design method of inertial interference self-compensating balance

This paper proposes a design method for a wind tunnel balance that can compensate for inertial interference, aiming to address the issue of short effective time in pulse wind tunnels. This method also tackles the problem of the force measuring system’s inability to dampen quickly and the coupling of inertia vibration signals with the output signal of the balance, which affects aerodynamic measurement accuracy. The method calculates vibration displacement at specific positions of the balance structure using its acceleration, constructs compensation signals based on the relationship between these displacements and balance outputs, and then superimposes them onto the balance signal for compensation. The effectiveness of this compensation method is evaluated through finite element simulations and wind tunnel tests. The results demonstrate that by compensating for the balance signal, there is a significant reduction in the amplitude of the inertia vibration signal, leading to an enhancement in the repeatability accuracy of the measurement. This compensation method effectively solves under-compensation or over-compensation issues when accelerometers are placed on complex models with various modes, where obtaining accurate compensation signals becomes challenging. It provides a new technical approach to improve aerodynamic measurement accuracy by mitigating inertial interference during pulse wind tunnel testing.

Wind Tunnel Balance Measurements of Bioinspired Tails for a Fixed Wing MAV

Bird tails play a significant role in aerodynamics and stability during flight. This paper investigates the use of bioinspired horizontal stabilizers for Micro Air Vehicles (MAVs) with Zimmerman wing-body geometry. Five configurations of bioinspired horizontal stabilizers are presented. Then, 3-component external balance force measurements of each horizontal stabilizer are performed in the wind tunnel. The Squared-Fan-Shaped Horizontal Stabilizer (HSF-tail) is selected as the optimal horizontal stabilizer that provides the highest aerodynamic efficiency during cruise flight while maintaining high longitudinal stability on the vehicle. The integration of the HSF-tail increases the aerodynamic efficiency by more than 6% up to a maximum of 17% compared to the other alternatives while maintaining the lowest aerodynamic drag value during the cruise phase. Furthermore, balance measurements to analyze the influence of the HSF-tail deflection on the aerodynamic coefficients are conducted, resulting in increased lift force and reduced aerodynamic drag with negative tail deflections. Lastly, the experimental data is validated with CFD-RANS steady simulations for low angles of attack, obtaining a relative difference on the measurement around 5% for the aerodynamic drag coefficient and around 10% for the lift coefficient during the cruise flight that demonstrates a high degree of accuracy in the aerodynamic coefficients obtained by external balance in the wind tunnel. This work represents a novel approach through the implementation of a horizontal stabilizer inspired by the structure of the tails of birds that is expected to yield significant advancements in both stability and aerodynamic efficiency, with the potential to revolutionize MAV technology.

Tehnika smanjenja opterećenja na unutrašnjim aerovagama tokom supersoničnih prelaznih pojava

Design of slender supersonic missiles requires a comprehensive experimental support in the form of wind tunnel data for a wide range of flight parameters (angle of attack and Mach number). However, in supersonic wind tunnel testing, the problem exists of transient loads at the times of the starting and stopping of the supersonic flow. In the environments of pronounced transient loads, characteristic of blowdowin wind tunnels like the T-38 of the Military technical Institute in Belgrade, it is necessary to provide control of the use of internal wind tunnel balances in the permitted design load ranges. The presented technique is related to the definition and implementation of a methodology for reducing the transient loads on wind tunnel balances in supersonic wind tunnel tests. By limiting the clearance between the model and its tail support (sting) to a magnitude which permits normal tests, but results in model-support contact during the excessive loads, part of the loads is transferred to the support sting, relieving the balance. The technique improves control over the wind tunnel test process, improves measurement accuracy and prevents damage to sensitive instrumentation (wind tunnel balances).

Overall and Local Wind Loads on Post-Installed Elevator Shaft of Existing Buildings

The glass curtain walls of post-installed elevator shafts in existing buildings can be damaged by local wind loads, and the serviceability of an elevator may be affected by excessive overall wind loads, especially in hurricane-prone areas. The overall and local wind load characteristics of elevator shafts with different arrangements (E-type, H-type, I-type) were studied using wind tunnel tests and computational fluid dynamics (CFD) numerical simulations. Firstly, high-frequency base balance wind tunnel tests of these elevator shafts with three arrangements were carried out to obtain the overall wind loads on the elevator shafts. Secondly, a CFD simulation was performed on the post-installed elevator shafts with three arrangements, obtaining the surface local wind pressure distribution of the elevator shafts under different wind directions. Finally, the wind-induced displacement responses of post-installed elevator shafts were analyzed. The results show that the aerodynamic interference of different elevator arrangements (E-type, H-type, I-type) and wind directions have significant effects on the overall local wind loads and wind-induced responses of the post-installed elevator, while the local wind loads on the area of the elevator door are less influenced by the elevator arrangement type than local wind loads on the surface and the overall wind loads of the elevator shafts. The results and conclusions may be helpful for developing the wind-resistant design of a post-installed elevator shaft.

Hypersonic aerodynamic force balance using temperature compensated semiconductor strain gauges

Metal foil strain gauges remain the state-of-the-art transducers for wind tunnel balances. While strain gauge technology is very mature, piezoresistive semiconductor sensors offer alternatives that are worth exploring to assess their unique benefits, such as better strain resolution and accuracy, which would enable balances to be designed with higher factors to safety and hence longer fatigue lifetimes. A new three-component balance, based on temperature compensated semiconductor strain gauges, is designed, calibrated and tested in a hypersonic low density wind tunnel. The static accuracy of the semiconductor balance is calibrated better than 0.3% FS, and the dynamic accuracy of the balance is established using a HB-2 standard model in a Mach 12 hypersonic flow. Good experimental repeatability is confirmed to be better than 2.5% FS, and the effectiveness of the balance is demonstrated by comparing the forces and moments of measured data with computational fluid dynamics simulations, as well as reference wind tunnel results under similar conditions.

Design Method of Three-Component Optic Fiber Balance Based on Fabry-Perot Displacement Sensor.

This article proposes a new type of three-component optic fiber balance based on Fabry-Perot displacement measurement technology based on the structure of the pulse wind tunnel balance. This paper systematically introduces the force measurement principle and design process of a three-component optic fiber balance and conducts relevant simulation analysis and experimental verification. The simulation results show that the Fabry-Perot sensor can achieve significant sensitivity to cavity length changes, and when used in existing balance structures, sensitivity gains can be achieved by changing the probe height without the need to modify the original structure of the balance. Finally, the feasibility of the design method was verified through calibration experiments: the optic fiber balance has high sensitivity and good linearity compared to simulation sensitivity, the error is less than 6%, and the calibration accuracy of each component is better than 0.13%, which is better than the existing traditional strain balance (0.37%). The pulse wind tunnel force measurement test has a short test time and a large model mass, and the balance needs to have a large stiffness to meet the short-term force measurement requirements. The introduction of more sensitive optic fiber balance force measurement technology is expected to solve the contradiction between the stiffness and sensitivity of force measurement systems.

Temperature Compensation of Wind Tunnel Balance Signal Detection System Based on IGWO-ELM.

The wind tunnel balance signal detection system is widely employed in aerospace applications for the accurate and automated measurement of aerodynamic forces and moments. However, measurement errors arise under different environmental temperature. This paper addresses the issue of measurement accuracy under different temperature conditions by proposing a temperature compensation method based on an improved gray wolf optimization (IGWO) algorithm and optimized extreme learning machine (ELM). The IGWO algorithm is enhanced by improving the initial population position, convergence factor, and iteration weights of the gray wolf optimization algorithm. Subsequently, the IGWO algorithm is employed to determine the optimal network parameters for the ELM. The calibration decoupling experiment and high-low temperature experiment are designed and carried out. On this basis, ELM, GWO-ELM, PSO-ELM, GWO-RBFNN and IGWO-ELM are used for temperature compensation experiments. The experimental results show that IGWO-ELM has a good temperature compensation effect, reducing the measurement error from 20%FS to within 0.04%FS. Consequently, the accuracy and stability of the wind tunnel balance signal detection system under different temperature environments are enhanced.

Present Status on Internal Wind Tunnel Balance Technology

No abstract

Titanium Wind Tunnel Balance Leveraging Additive Manufacturing

Titanium force transducers have among the highest strength–to–Young’s modulus ratios of common transducer materials. This can be leveraged to increase the sensitivity of the transducer relative to other materials or to increase the structural safety factor of the transducer for a given sensitivity. It is surprising then that no titanium wind tunnel balances have been reported in the literature or produced by NASA. With the increasing maturation of additive manufacturing technology, this work explores additively manufacturing a titanium wind tunnel balance. Additive manufacturing of the balance was completed in less than 1 week. Then, several secondary finishing and machining operations were performed to finalize the balance geometry, and it was strain gaged and calibrated. Calibration results confirm that the titanium balance has approximately 68% higher output compared to a steel balance of identical geometry and applied load. Based on the calibration results, hysteresis, pure error, and accuracy of the titanium balance are reported. These performance characteristics are comparable to conventionally manufactured state-of-the-art steel balances.

An improved rigid rod method for wind-induced swing response prediction of heavily iced transmission lines

Wind-induced swing of overhead transmission lines may cause flashover and line tripping, which will greatly jeopardize the normal operation of power transmission system and give rise to great economy loss. This paper analyzed a swing flashover accident on a practical high-voltage (HV) transmission line and calculated the swing angle of the suspension insulator string using the rigid rod method recommended by the Chinese standard. The aerodynamic coefficients of the iced HV transmission lines were obtained through high frequency force balance wind tunnel tests. An improved rigid rod method was then developed for predicting wind-induced swing response of heavily iced transmission lines in mountainous terrain under severe icy weather conditions. Finally, the improved rigid rod method was also extended to deal with general transmission lines with light ice accretion or regular ice shapes.

High pitch sweep rates in supersonic wind tunnel tests

A typical test in a high-Reynolds-number blowdown wind tunnel, such as the VTI T-38 facility in Belgrade, involves a model sweep in a range of angles of attack. Increasing pitch sweep rates can bring two-fold benefit. The angle-of-attack range can be extended in tests at high stagnation pressures with limited run time, or test time can be shortened for the desired angle-of-attack range in other tests, in both cases bringing significant energy savings. The potential effect of the increased pitch rates on measurement data in the VTI T-38 wind tunnel was investigated in a recent supersonic wind tunnel test campaign of a hypervelocity standard model. The Mach 2 tests were performed at pitch rates of 2, 3, 4, 8 and 12° per second. While the wind tunnel balance measurement data were not seemed to be affected, the base pressure data revealed a high dependency on changing pitch sweep rates. It was found that the high lag in pressure piping affected the measured base pressure and, consequently, the forebody axial force, which is an important fact to know when dealing with ‘base drag corrected’ supersonic data. The conclusions derived from the Mach 2 tests have been verified in the additional Mach 4 tests. The verification tests confirmed that, for the currently employed base pressure measurement system, reliable measurement results can be obtained in wind tunnel tests by applying pitch rates up to 3° per second. The recommendations for modifications of the base pressure measurement system have been given.

Experimental investigation and analysis on the cross flow characteristics over inline tube bundles with S/D=1.875

Helical tube bundles are usually used for steam generators (SGs) and intermediate heat exchangers (IHXs) of High Temperature Gas-cooled Reactors (HTGRs). The thermal hydraulic performance of SGs/IHXs is dominated by the cross flow convection over tube bundles in the primary side. Though cross flow over tube bundles were widely investigated as a basic flow pattern, few of them focused on tube bundles with 1.4-1.6<S/D<2-2.2 (S, tube pitch, D, tube diameter). However, the designs of HTGR SGs or IHXs prefer smaller tube pitches due to the relatively low heat transfer coefficient. In present investigation, velocity distribution, surface pressure distribution and drag/lift force of cross flow over tube bundle with S/D=1.875 are measured and investigated thoroughly in a wind tunnel. The velocity fields are measured using time resolved PIV and split hot film methods. The surface pressure distribution is measured using dynamic pressure transducers. The drag and lift forces are measured using high frequency wind tunnel balance. Due to the friction drag introduced by the bounded wall, the time averaged velocity in the flow passage next to the bounded wall is smaller than those in the middle flow passages. The surface pressure distributions between the impinging point and the separation point on tubes are characterized by the acceleration effects when fluid flowing into the region between neighboring tubes in the same row. The increasingly significant pressure recovery after separation results in the decreasing tendency of flow resistance coefficient with Reynolds number (Re) increment. A main frequency corresponding to Sr=0.114 (Sr, Strouhal number) is observed in both surface pressure and velocity spectrums, indicating that the vortex shedding induces the turbulent wake swinging. The turbulent wakes after tubes in the same row swing with no phase lag, while turbulent wakes of tubes in neighboring rows swing with a phase lag around 0.33 periods. By using an improved phase average method, phase averaged turbulent statistics and instantaneous coherent structures are obtained. The main flow region swings with vortex shedding. The region with larger 〈u′u′〉 (u′, fluctuating component of stream wise velocity) mainly locates at the shear layer, swinging with the vortex shedding. The region with larger 〈v′v′〉, (v′, fluctuating component of transverse velocity) mainly locates in front of the tubes, where transverse flow sweeps the upwind side of the tubes.

New Flow-Through Wind-Tunnel Balance for Retropropulsion Testing

Supersonic retropropulsion, or the use of retrorockets starting at supersonic conditions, is an enabling technology for large-scale human Mars lander vehicle concepts. A new flow-through balance was designed and fabricated to support an upcoming retropropulsion test campaign at the NASA Langley Research Center Unitary Plan Wind Tunnel. The balance employs membranes in place of bellows, which have historically been used, and passes the flow-through load-bearing parts of the balance. Flow through the balance is optimized via the addition of flow guides, which are manufactured as separate parts, and attached to the balance. Calibration of this new flow-through balance demonstrates that it can suitably characterize the load and pressure ranges it was designed for, which provides aerodynamicists a new tool to directly measure supersonic retropropulsion performance of powered descent vehicles.

Drag reduction in cylindrical wake flow using porous material

Due to its unique pore structure, porous materials have the potential to be used in the fields of acoustic noise reduction and flow drag reduction control. In order to study their effects and mechanism of drag reduction on the flow around a circular cylinder, experiments are conducted in a low-speed wind tunnel with low turbulence intensity. The drag forces acting on a circular cylinder model are measured using wind tunnel balance when porous materials with different permeability are applied within different intersection angles on the trailing-edge and leading edge, and the flow fields are visualized with a particle image velocimetry system with high time resolution. The method of dynamic mode decomposition (DMD) is also used for reduced-order analysis of the vorticity field in the wake of the cylinder. The measured drag forces and wake flow fields are then compared with those of a smooth cylinder, and the results show that porous materials laid on the trailing-edge can reduce drag, when a porous material with 20 pores per inch is laid within 270° on the leeward side, the best effect of the drag reduction ratio of 10.21% is reached. The results of flow visualization indicate that after the porous material is applied, the vortex region in the wake of the cylinder is expanded; both the frequency of vortex shedding and the magnitude of vorticity fluctuation decrease; the Reynolds-shear-stress decreases significantly, and both indicate that vorticity is dissipated earlier. The results of DMD analysis show that porous materials can effectively relax the energy of vortices in different modes.

Application of a technique for reducing supersonic starting loads on internal wind tunnel balances

Higher than anticipated supersonic starting loads in the VTI T-38 wind tunnel were a consequence of higher than designed minimum start pressures. Initially, high starting loads limited the usefulness of the facility at high supersonic Mach numbers. Over time, it was established that several procedures and techniques had to be used cooperatively to diminish the problem. One of the techniques is the establishment of additional points of contact of the model and the support sting for the duration of the starting loads, so that a part of the aerodynamic loads is transferred to the sting through the added path, relieving the sensitive force balance. Some applications of this technique require dedicated mechanisms to operate, which was impractical for the VTI T-38 wind tunnel. To the contrary, the implementation of the idea in the VTI T-38 wind tunnel relies on using suitable inserts to make the clearance between the model and the sting such that model-sting contact is forced to occur during the start and stop phenomena, but not during the measurement phase of the test. For typical model configurations, transient starting and stopping loads can be reduced by more than 50%.

System modeling and control system performance optimization of magnetic suspension and balance system

A magnetic suspension and balance system (MSBS) is a kind of aerodynamic test equipment for aircraft. It uses levitation control technology to replace the mechanical support and strain measurement technology in traditional wind tunnel balance. It has the advantage of no model support to interfere with the wind tunnel flow field and make the test environment more realistic, but the MSBS has technical difficulties to be solved. It is difficult to control and stabilize because it has a large suspension gap, nonlinear magnetic field, a long-acting distance of magnetic force, and uses a visual sensor to feedback the position of the controlled object that it has signal noise. At present, there are few studies on the suspension control method and suspension performance of MSBS. In this paper, in order to solve the above technical difficulties, the vertical control of the MSBS is taken as the research object, the electromagnetic field of the vertical coil is modeled and analyzed. A new control algorithm based on a fast-tracking differentiator (FTD) is proposed to solve the signal noise problem in MSBS. The influence of various factors in the control link on the suspension system is analyzed, and its optimization is carried out to improve the performance of the suspension system.

Analysis of the propellers-airframe interaction of the light transport aircraft

In the design of multi-engine aircraft, one of the important issues is the interaction between the propellers and airframe configuration components, especially in take-off and go-around procedure modes. Modern propeller-driven aircraft concepts in the pulling configuration are characterized by a high disk loading and an increased number of propeller blades used to increase cruising speed and reduce excessive noise. The first problem arising due to high disk loading is the direct impact of forces by operating propellers (thrust, normal force) on fixed-wing stability, especially at angles of attack different from a zero value. The second one involves a high-energy level of the propeller slipstream, having a significant indirect impact on the aircraft’s aerodynamics, stability and controllability. This impact is primarily associated with the interaction of propellers slipstream with other aircraft’s configuration elements. The complexity of taking into account the slipstream-wing interaction and other airframe components stipulated the application of experimental methods to study the problems of propellers – airframe interaction while designing propeller-driven aircraft configurations. This article presents an analysis of the experimental studies results of the operating propellers- airframe interaction for a light twin-engine transport aircraft. The aerodynamic aircraft’s configuration is executed using the conventional pattern of a high-wing and the carrier-on deck type empennage. The high-lift wing device is a fixed-vane doubleslotted flap. The wind-tunnel tests of the model in the cruising, takeoff and landing configurations were carried out in TsAGI lowspeed wind-tunnel T-102. Measurement of forces and moments, acting on the model, was performed by means of an external sixcomponent wind-tunnel balance. Measurement of forces and moments, acting on the propeller, was conducted using strain gauge weighers installed inside the engine nacelles of power plant simulators. The simultaneous combined use of external and internal balances allowed researchers to determine the direct and indirect contribution of operating propellers to the model longitudinal aerodynamic characteristics under variation of loading factor B ranging from 0 to 2.

Wind-resistant structural optimization of supertall buildings based on high-frequency force balance wind tunnel experiment

The high-frequency force balance (HFFB) wind tunnel test is one of the most widely used techniques to estimate wind effects on supertall buildings. This paper introduces a wind-resistant optimal design method of supertall buildings based on HFFB wind tunnel experiments. Based on the HFFB wind tunnel test and wind-induced vibration analysis, it provides a reliable and efficient method for structural optimization of supertall buildings with different types of structural members and multiple constraints. Taking advantage of the classic linear mode shape assumption of the HFFB technique, this paper also presents an alternative routine that transforms all different types of constraints into a uniform frequency constraint, which significantly reduces the computational intensity while still offers acceptable accuracy of optimization results. Moreover, the sensitivity of the optimization results to the adopted basic wind pressure value is discussed. The total material cost of the optimized structure increases exponentially with the increase of the basic wind pressure, indicating that the structural costs of supertall buildings are quite sensitive to the adopted basic wind pressure for structural design and its value should be precisely determined by the wind climate analysis of the construction site.

Calibration design evaluations through computational analysis and investigation of a six-component wind tunnel balance

Calibration design evaluations through computational analysis and investigation of a six-component wind tunnel balance

Numerical Computation and Analysis of Electromagnetic Field in Magnetic Suspension and Balance System

The magnetic suspension wind tunnel balance (MSBS) is an entirely new device for aerodynamic measurement, and it makes the best of the electromagnetic force to suspend the aircraft model in the wind tunnel without contact. Compared with conventional wind tunnel balance, it absolutely abandons the model support and airflow interference. Therefore, the aerodynamic measurement environment is more authentic and the aerodynamic measurement results are more accurate. The electromagnetic field in MSBS plays a major role in bearing the force of wind. The numerical computation and finite element numerical analysis are performed to investigate key factors of electromagnetic force under different conditions. The calculation results based on finite element method (FEM) have revealed that the diameter and the spacing of of the axial coil, the number of segments and the pitch angle of the suspension model are key factors of electromagnetic force. Based on the above key factors, the structure of the magnetic suspension balance is optimized to maximize the electromagnetic force under multiple constraints.