Ixcán: las consecuencias cotidianas de la guerra

The literature that is relevant to the wing flow has previously been reviewed by Zerihan [8] and Mahon [12]. Therefore it has been decided to refer to these works, instead of to review this topic as extensively as the wheel literature. The equivalent to tables 1.3 and 1.4 can be found in Mahon’s thesis [12], summarizing the experimental and computational literature for inverted wings in ground effect. Furthermore it is worth mentioning that the typical flow features and mechanisms that occur for the specific isolated wing that has been used within the current research are analyzed in section 3.2 of this thesis.

1.3.1 General wing characteristics

Since it is impossible to look at all subject areas concerning wing flow, only some specific topics will be covered. General knowledge of wing aerodynamics is assumed and further information can be found in [8, 12]. Multi-element wings can be used to increase the generated lift and / or to postpone stall to a higher angle of attack. Smith [71] defined five effects of slots for multi-element wings that influence the pressure distribution over the elements. A multi-element wake is characterized by confluent boundary layers, where the wake of the upstream element interacts with the downstream element boundary layer on the suction side. Several 2D numerical studies have been performed to analyze this viscous feature [12]. Petrov [72,73] found that bursting of the wake of an upstream element could lead to a stable off-surface separated region, which limits the lift of the downstream element(s). This lift limiting mechanism creates a gradual stall with increasing incidence, in contrast to the more violent leading edge stall. Mahon [12] concluded that this 2D mechanism also limited the downforce experienced for a wing in ground effect with ride height reduction. He could however not find any conclusive experimental proof for this statement.

Reynolds effects play an important role in multi-element airfoil aerodynamics. In general an increase in Reynolds number will lead to beneficial separation characteristics, because the boundary layers become more resistant to adverse pressure gradients. However for multi-element wings adverse Reynolds effects may occur, which are a result of the reduction in boundary layer thickness with increasing Reynolds number and lead to a

Introduction and literature review

disadvantageous increase in the effective slot gap between the elements [12]. Mahon’s table of previous numerical studies of multi-element airfoils and wings reveals that both RANS and URANS methods have been used in combination with a large variety of turbulence models. A final aspect of wing aerodynamics that is essential to the subject of wing - wheel flow interaction are the tip vortices that result from the finite pressure differences between the wing (and endplate) surfaces. A large amount of literature is available on the characteristics and downstream development of these tip vortices (i.e. see [74]); further discussion of vortices in general can be found in section 1.4.1.

1.3.2 Inverted wing in ground effect

Racecar (front) wings usually consist of multiple elements in combination with compo- nents to control the flow, such as endplates, endplate feet and in some cases Gurney flaps. The suction surfaces of the elements are directed downwards in order to create downforce. Endplates [75] provide a means to maximize the wing performance for fixed wing dimen- sions by separating the suction and pressure surfaces of the wing at the tip. This results in a finite pressure difference between the top and bottom at the spanwise extremities of the wing and thus increases the downforce [12]. Endplate feet, which are outward horizontal extensions at the bottom of the endplate, introduce an additional downforce enhancing mechanism in the form of an extra vortex underneath each of the feet, while the other lift enhancing mechanisms get magnified as well [12]. Finite trailing edges [12] and / or Gurney flaps [76, 77] can be used to generate more downforce by creating a finite pressure difference over the trailing edge. The latter is a short strip fitted perpendicular to the pressure surface along the trailing edge of a wing element. Alternate vortex shedding can occur behind the blunt trailing edge or Gurney flap, however this phenomenon ceases to exist when separation from the suction side starts taking place [77].

Several options are available to model a wing in ground effect. However only the use of a moving ground installation will lead to correct results [8], while the other possibilities of using a fixed ground with or without suction or blowing, and the reflection method using mirrored models, all have their shortcomings. A wing in ground effect can produce significantly higher downforce compared to when it is placed in freestream conditions. The downforce increases with reducing ground clearance until it reaches a maximum value after which flow separation occurs. Zerihan [8] introduced the analogy between a wing in freestream with increasing incidence and the same wing in ground effect with reducing ride height. The trend with parameter variation is qualitatively very similar for both

Introduction and literature review

situations, although the suction peak and resulting downforce reach higher values for the ground effect case. The main difference is that the freestream stall occurs more abruptly due to the instabilities caused by the separation from the trailing edge moving upstream, whereas the ground effect stall is more gradually. The constraining of the flow by the moving ground prevents the separated region to move upstream and therefore separated regions can be sustained in a more stable way in ground effect. Therefore separation is not the sole downforce limiting factor for wings in ground effect, in contrast to for wings in freestream, whose maximum downforce occurs in general just before flow separation takes place.

Flow field studies of the wing tip vortices [10, 12] revealed that another governing force enhancement mechanism for a wing in ground effect is related to the generation, dilution and breakdown of the wing tip vortices. The presence of a wheel downstream of the wing, during the current research, will influence the path and state of the tip vortices and therefore the downforce that the wing produces. The wake and vortices, that are induced by the wing, will on the other hand influence the pressure distribution on the wheel as well. Understanding of the interaction between these flow fields will be essential in obtaining insight into the aerodynamics of the combined components.

Wings in freestream can experience hysteresis effects, which make the flow features depending on the direction of the parameter change. An abruptly stalled wing will for example not immediately return to the attached flow case when the angle of attack is re- duced. Further reduction of the incidence is required before the pre-stall flow is restored. In a similar way a diffuser body in ground effect displays hysteresis effects with ride height reduction [14]. Mahon [12] was the first to discover hysteresis effects for the wing in ground effect, showing that the force coefficients at low ride heights were dependent on the direc- tion of ride height change. It is important to realize that this hysteresis effect is not a time dependent, dynamic result, like the downforce changes due to instantaneous movement of the wing as simulated by Moryossef [78], but a sustainable difference between the increas- ing and decreasing ride height variation. For the hysteresis results the measurements have been taken after a settling period at a static constant wing ride height.

The summary of numerical simulations for wings in ground effect by Mahon [12] shows that only a limited number of results is available in literature. Most of these studies concern 2D airfoils, including the recent contribution by Mahon [11]. Although Mahon also did some preliminary simulations for a 3D wing in ground effect [12], these results have not been published. From this it can be concluded that there is a severe lack of

Introduction and literature review

literature on 3D wing in ground effect simulations, especially with respect to the influence of the ride height on the flow features and mechanisms. Such a study could result in better understanding of the flow physics for a wing in ground effect and would add to the knowledge available from 2D simulations, in which the tip effects are not simulated [9,11]. Finally it needs to be mentioned that Guilmineau [79] successfully simulated hysteresis effects resulting from deep dynamic stall for a 2D wing in freestream using a URANS approach.