Journal of Aerospace Mechanics
Year:2009 | Volume:4 | Issue:4 (14)|Papers Count:8

Lean premixed combustion is widely used in recent years as a method to achieve the environmental standards with regard to NOx emission. In spite of the advantages, premixed combustion systems with equivalence ratios less than one are susceptible to combustion instability. To study the lean combustion instability experimentally, one premixed combustion setup, equipped with reactant supplying system, is designed and manufactured. In this research, gaseous propane is introduced as fuel and experiments are performed at nearly atmospheric pressure with equivalence ratios within the range of 0.7 to 1.5 and fuel mass flow rates of 2 to 4 gr/s. It has been observed that the combustion chamber of the gas turbine becomes unstable when equivalence ratio is less than one. To distinguish the combustion instability, through various operating conditions, probability density functions, spectral diagrams, space distribution of pressure oscillations, along with Rayleigh criteria are employed. Accordingly, equivalence ratio effects in stabilizing the unstable combustion system are investigated experimentally. Moreover, convective delay time is calculated for all experiments and the results are compared with Rayleigh Criteria, showing relatively good agreements. Finally, stability limits are identified, based on inlet mass flow rate and equivalence ratio.

In this research, digital shearography as a non-destructive technique is applied to determine opening mode stress intensity factor in a specific cross-ply composite specimen with edge cracks. Optical set-up with two in-plane illumination sources is used to determine purposed strain variables and consequently obtain the first mode stress intensity factor as a result of applying linear elastic fracture mechanics (LEFM). The results of shearography technique are some fringes which contain the information needed to obtain the strain variables. Processing these images numerically will result the parameters related to the variables of strains. The results are compared with those of applying empirical equations and hence the ability of this method is evaluated.

Equations of Large Eddy Simulation are obtained by applying low pass filter on the Navier-Stokes equations. The turbulent modeling in LES depends on the type of the filter function used. In this paper, two explicit filtering methods are compared, using a fully developed turbulent channel flow. First, it is simulated by explicit filtration method applying the smooth Laplace filter. In this case, the subfilter stress term is added to the subgrid model. Good agreements are observed in comparison with Kim's DNS results. Second, in order to consider the effects of closeness of explicit filter shape to the cut-off filter, three different filter types were used for the flow simulation. Finally, the abilities of each approach were investigated. Moreover, the regions of boundary layer being more different from each approach are specified.

In this paper, a new analytical model for penetration of deformable blunt projectile into metallic targets is developed, using the plastic stress waves theory and momentum equation. During the penetration process, failure of target plate is assumed to be plugging with dishing of target plate and projectile deforms into mushroom shape. In this model, penetration stages change with relation to old models and penetration stages are classified in nine stages. Material projectile and target are defined as rigid-linear work hardening and rigid-perfectly plastic, respectively. For each stage, equations of motion for projectile and target are derived. Equations for projectile erosion and deformation, plug formation and ejection, and dishing of the target are solved instantaneously. The analytical results are in good agreements with the existing experimental ones.

In this paper, pure proportional guidance in 3-D space is first explained with a new perception. The main idea is based upon the distinction between angular rate vector and rotation vector conceptions. In this innovation, the emphasis is based upon the selection of line of sight coordinates and comparison between the two available views for choosing this system. Then, using an additional term, an improvement to this law is made. This term compromises a cross range compensator, which is used to provide first fluctuations for obtaining convergent estimates of state variables. Then, a state-space description within the improved spherical coordinate system has been offered. The available measurements in this system have been chosen with regard to the considered practical points. Then, the issue of range-to-target estimation is proposed and some non-linear filters, such as extended Kalman filter, unscented Kalman filter, particle filter, EKF particle filter, and UKF particle filter in the modified spherical coordinates have been used. Simulations indicate that the proposed tracking filters in conjunction with the dual guidance law are able to provide the convergence of the range estimation for both maneuvering and non-maneuvering targets.

Natural draft dry cooling towers are amongst the usual towers mostly used in arid areas. Surrounding conditions, such as wind blowing, is an important factor in tower operation which decreases the efficiency of cooling towers. To study this effect numerical, experimental and field methods could be used. Each of these methods has its own advantages and disadvantages but they complement each other. In this article, the effect of wind on natural draft dry cooling tower performance was studied both numerically and experimentally. Effect of different wind break walls arrangement on the cooling tower was studied to address the restriction of this unfavorable effect. The fluid flow inside and around these towers was modeled using finite volume method and in experimental investigation, the pressure distribution at the inlet of cooling towers was studied, using the wind tunnel at Shahrood University of Technology. Results indicate that changes in the external shape of natural draft dry cooling towers improve their performance under cross wind condition and the eight radial wind break walls have the best performance in reducing the unfavorable effects.

A machining force model for ultrasonic vibration-assisted turning (UAT) has been developed in the present study. For this purpose, extensive full-factorial experiments were carried out and the results were analyzed statistically. Various influential parameters, such as cutting speed, feed rate, depth of cut, and ultrasonic vibration amplitude were taken into account. The proposed model was selected from among four different regression models. The condition for the robustness of the proposed model was then investigated. Several experiments were also carried out on conventional turning and the results were analyzed and compared with those obtained during ultrasonic vibration-assisted turning. It was shown that the quadratic polynomial model can better estimate the machining force. The latter experiment with a vibrating tool is unconditional and considerably lower than that created in conventional turning. The continuous chip being formed in UAT is partially responsible for more efficient machining (from the view point of ductile regime of machining) compared to conventional turning (CT).